Habitat selection by song thrushes in stable and declining farmland populations

Journal of Applied Ecology

2004

41

, 275–293

© 2004 British Ecological Society

Blackwell Publishing, Ltd.

Habitat selection by song thrushes in stable and declining farmland populations

WILL J. PEACH, MATTHEW DENNY, PETE A. COTTON*, IAN F. HILL, DEREK GRUAR, DAVID BARRITT, ANDREW IMPEY and JOHN MALLORD

Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2DL, UK

Summary

1.

Losses of farmland birds from the wider countryside have become a major con-servation issue in the UK and Europe. Song thrush

Turdus philomelos

populations inlowland rural Britain declined by approximately 70% during 1970–95, most severelyon intensively managed arable farmland. Comparison between a stable population onmixed farmland and a rapidly declining population on arable farmland revealedfewer nesting attempts each summer by birds in the declining population, and annualproductivity was insufficient to maintain local population density. Inadequate foodresources were the most plausible cause.

2.

We compared breeding season habitat selection (using radio-telemetry) and earth-worm availability (a major component of summer diet) for song thrushes in the sametwo farmland populations.

3.

Territory settlement in the mixed farmland landscape involved the selection of fieldboundaries and woodland and the avoidance of arable crops. Field boundaries andgardens were selected in the arable landscape, while arable break crops and small areasof woodland were avoided.

4.

Habitat selection (intensity of usage) did not change through the breeding seasonand did not differ between study areas. Scrub, woodland edge, wet ditches and bare soilin gardens were preferred foraging habitats, while cereals were avoided.

5.

Habitat utilization (amount of usage) differed markedly between study areas. Woodlandand grassland accounted for 53% of all habitat usage in the mixed farmland landscapecompared with just 13% in the arable landscape. Gardens and arable crops were moreheavily utilized in the arable landscape, accounting for 58% of all usage compared with22% in the mixed landscape.

6.

Earthworm availability declined markedly between April and June as surface soilsdried out. Lower earthworm availability in the arable landscape was associated withmore rapid and pronounced drying of surface soils.

7.

Synthesis and applications.

Lack of woodland and grassland, and the faster drying ofsurface soils in the arable landscape, combined to limit the availability to thrushes of keysummer invertebrate prey. Loss of hedgerows, scrub and permanent grassland with live-stock, and the wide-scale installation of under-field drainage systems, have probably allcontributed to the decline of song thrushes on UK arable farmland. New agri-environmentmeasures may be needed to provide the nesting cover adjacent to invertebrate-richdamp soils that song thrushes require to sustain annual productivity.

Key-words

: arable farmland, earthworms, farmland birds, field drainage, grassland,log-linear models, radio-telemetry, woodland edge


(2004)

41

, 275 –293

Correspondence: Will Peach, Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2 DL, UK ([email protected]).*Present address: School of Biological Sciences, University of Plymouth, Plymouth PL4 8AA, UK.

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© 2004 British Ecological Society,


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,275–293

Introduction

Declining farmland bird populations have becomea major conservation issue in the UK and Europe (Fuller

et al

. 1995; Donald, Green & Heath 2001), resultingin a suite of studies aiming to identify key environ-mental factors driving population declines (Aebischer

et al

. 2000). Approaches include broad-scale studies ofspatial and temporal variation in bird densities anddemography (Peach, Siriwardena & Gregory 1999;Siriwardena

et al

. 2000) and fine-scale investigations ofbird density and performance in relation to contrastingagricultural management (Wilson

et al

. 1997; Brickle

et al

. 2000). Another approach is to compare and con-trast demographic and/or ecological traits in populationsexhibiting differing population trends (Green 1994).This may be particularly appropriate when the plausiblecandidate environmental factors are diverse.

Song thrushes

Turdus philomelos

Brehm experienceda rapid population decline in lowland Britain betweenthe mid-1970s and early 1990s, since when breedingdensities have remained relatively stable at a reducedlevel (Baillie

et al

. 2001). The decline has been greateston farmland (approximately 70% of pairs lost) but alsopronounced in lowland woodlands (

c.

50% of pairslost). Densities of breeding song thrushes are now lowon most intensively managed arable farmland, with themajority of remaining territories associated eitherwith gardens or woodland (Mason 1998). Extensivedemographic data indicate no long-term changes innesting success per attempt (Baillie

et al

. 2001) buthighlight reduced survival among fully grown birdsduring their first year of life as one likely demographicmechanism driving the population declines of the late1970s and mid-1980s (Thomson, Baillie & Peach 1997).Severe winter weather could not account for the popu-lation decline.

A diverse range of environmental factors may havehad negative impacts on farmland song thrushes. Theseinclude loss and degradation of key feeding and nestinghabitats (Mason 1998), increased depredation of nestsand fully grown thrushes from increasing populationsof avian and mammalian predators (Thomson

et al

.1998; Paradis

et al

. 2000; Stoate & Szczur 2001), drysummer weather (Mason 1998) and direct and indir-ect effects of pesticides (Campbell

et al

. 1997). In anattempt to narrow this range of candidate environ-mental factors, in 1995 the Royal Society for the Protectionof Birds (RSPB) initiated a comparative ecologicalstudy of two farmland song thrush populations ex-hibiting contrasting demographic trends. A populationthat appeared to have been stable since the mid-1970swas located on mixed farmland in west Sussex, Eng-land, and a rapidly declining population was identifiedon intensively managed arable farmland in Essex, Eng-land. Annual productivity was much lower in thedeclining population (1·8 and 2·5 fledged young pair

−

1

in 1996 and 1997, respectively) than in the stable popu-lation (4·8 and 4·5 fledged young pair

−

1

) (Thomson &

Cotton 2000; R. E. Green, unpublished data). Produc-tivity in the stable population was about the levelrequired to offset annual mortality losses typical ofstable thrush populations (Thomson, Baillie & Peach1997; Thomson, Baillie & Peach 1999), while that in thedeclining population would result in annual popu-lation declines of approximately 22–28%, close to theactual declines of 19% during 1996–97 and 26% during1997–98. In the absence of immigration, unrealisticallyhigh annual survival rates (approximately 70% for adultsand 63–88% for first-years) would have been necessaryto maintain population levels given these low levels ofreproductive output.

The difference in productivity between the two thrushpopulations was caused mainly by fewer nesting attemptsper pair per year, rather than by lower success rates pernesting attempt. While initiation of first clutches occurredapproximately simultaneously in the two populations,intervals between subsequent nesting attempts weresignificantly longer in the declining population, withfewer nesting attempts, particularly during the latterpart of the breeding season in June and July (Thomson& Cotton 2000; R. E. Green, unpublished data). It iswell known that the timing of egg laying and the number(or volumes) of eggs laid is limited by female bodyreserves (Jones & Ward 1976) and by dietary intake ofprotein (Bolton, Houston & Monaghan 1992; Houston1997). It is likely that the propensity of birds to lay sec-ond or third clutches in any breeding season is similarlyconstrained by nutritional factors (Verboven, Tinbergen& Verhulst 2001), and seasonal shortages of key dietaryresources are an obvious candidate hypothesis account-ing for the lower productivity in the declining thrushpopulation.

The aim of this study was to compare breedingseason habitat usage and food resources available tothrushes in the stable and declining populations. Weaimed to test two hypotheses: (i) song thrushes in theintensively managed arable landscape lacked key food-rich habitats that were present and heavily utilized bysong thrushes in the mixed landscape; (ii) seasonaldeclines in the availability of key prey were more pro-nounced in the arable landscape.

Although several previous studies have consideredpreferences of song thrushes for particular field typesor field boundaries, this is the first to assess the relativeusage of all available habitats.

Methods

The two lowland rural study areas were characterizedby farmland interspersed with small villages and iso-lated settlements. The first study area was on mixedfarmland close to the South Downs in west Sussex, UK,centred on a long-term Common Birds Census (CBC)plot at Graffham village (50

°

57

′

N, 0

°

41

′

W). The songthrush population at this site had remained relatively

277

Habitat selection by farmland song thrushes



,

41

,275–293

stable (seven pairs in 1969 and six pairs in 1985). Thesecond study area was in an intensive arable landscapenear Chipping Ongar, Essex, UK, centred on a CBCplot (51

°

41

′

N, 0

°

12

′

E) where song thrush numbers hadfallen from 12 to two pairs between 1989 and 1997.

The mixed farmland study area in Sussex coveredapproximately 560 ha comprising mainly grazed grass-land (cows, horses and sheep, 26% of land area), arablecultivation (23%), field boundaries (16%), woodland(11%), ungrazed grass (set-aside, silage and hay, 9%)and gardens (12%). In contrast, the 780-ha Essex studyarea was dominated by mainly autumn-sown arablecrops (60% of land area), with smaller areas of ungrazedgrass (set-aside and hay, 13%), field boundaries (12%)and gardens (8%), with much smaller areas of grazedgrassland (horses only, 3%) and woodland (2%). Scrubcomprised 0·2% of the mixed study area and 0·7% ofthe arable study area. Densities of territorial songthrushes were higher in the Sussex study area (26·4 ter-ritories km

−

2

in 1996 vs. 7·6 territories km

−

2

in Essex). Inthe years immediately preceding this study, our ownweekly surveys of singing and breeding activity indi-cated that the numbers of territorial pairs remained rel-atively stable in the Sussex study area (130 territories in1995, 150 in 1996 and 121 in 1997) but declined markedlyin the Essex study area (58 territories in 1996, 47 in 1997and 35 in 1998).

-

Habitat usage by adult song thrushes was studied dur-ing the breeding seasons of 1999 and 2000 using radio-telemetry. Adult thrushes were trapped in mist-netsand fitted with Biotrack TW-4 button cell transmitters(Biotrack Ltd, Wareham, UK), weighing approxi-mately 1·5 g (

c

. 2% of body mass). Initially (in 1999only) transmitters were glued to one of the central tailfeathers, but after problems with tail feathers and tagsbeing shed from the bird we used a back-pack harnessthat has been shown not to influence breeding param-eters in blackbirds

Turdus merula

(L.) and song thrushes(Hill, Cresswell & Kenward 1999a). These harnessesnever fell off birds within the lifetime of the transmitterbattery, but were designed to fall off birds within 6–12 months of attachment.

The location of each tagged bird was determinedup to four times each day by triangulation (White &Garrott 1990). In order to maximize the detection ofcases where thrushes moved during the triangulation pro-cedure, ‘fixing’ involved two observers simultaneouslyobtaining a first pair of bearings on the tag. Oneobserver then moved (aiming to achieve a second bear-ing that differed from the first by

c

. 70–110 degrees)before a second pair of simultaneous bearings wastaken. Moving birds were identified as those where thebearing from the stationary observer changed or wherethe three main bearings (first pair plus the second fromthe moving observer) did not cross each other to forma triangle. In these cases (19% of 4007 fixes considered

here), fix location was taken as the intersection of thefirst pair of bearings. Otherwise, fix location was takenas the central point of the triangle formed by the threebearings. All fix locations were recorded as an eight-figure grid reference (i.e. to the nearest 10 m by 10 mintersection). The mean area of the fix triangles (i.e. fixprecision) was 0·048 ha in the mixed study area and0·044 ha in the arable study area.

Tagged thrushes were located between 06.00 and20.00 h (British Summer Time) between 19 Februaryand 3 August in 1999 and 2000. There were only twoinstances when both members of a breeding pair weretracked simultaneously, and we made no attempt toallow for this minor lack of independence in the data.We excluded all records from one apparently unpairedmale in the mixed study area that roamed over an unu-sually large area (2·5 times larger than the home rangeof any other tagged bird) and sang frequently through-out the day. In order to maximize the proportion offix locations associated with foraging, we excludedall records of non-provisioning birds during the 3days after capture (to allow acclimatization to the tag;provisioning birds appeared to continue to feed chicksnormally following tagging), of females within 20 m ofthe nest during the incubation period, and of malesknown to be singing at the time of fixing. These exclu-sions left available for analysis 1989 fix locations from46 individual thrushes in the Sussex study area (median35 per bird, range 5–142), and 1972 locations from 40individuals in the Essex study area (median 40, range8–155). Although these data include an unknown pro-portion of non-foraging activities, foraging is likely tohave been the main activity away from the nest duringperiods when chicks or fledglings were dependent onparents for food (Hill, Cresswell & Kenward 1999b).Usage of habitats like scrub or woodland might partlyreflect their importance as sources of cover for nesting,predator avoidance or loafing.

Home ranges were of interest in their own right andwere used to define habitat available to individual birdsfor use in habitat selection analyses. Inspection of thefix data (radio locations) indicated that adult thrushestended to range more widely when they were not attend-ing active nests or recently fledged young. We thereforedefined nesting home ranges as minimum convex poly-gons (MCP) incorporating all fixes collected duringperiods when adult thrushes were regularly attendingnests. For female thrushes, this included periods of nestbuilding, egg laying, incubation, chick rearing andfeeding fledged young up to 10 days after leaving thenest. For males, this only included periods of chickrearing and feeding fledged young up to 10 days afterleaving the nest. Fixes collected when adult thrusheswere not engaged in these activities were used to defineglobal home ranges (also 100% MCP based on all thereliable fixes). Nesting home ranges (

n

=

54) were usually,

278

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,275–293

but not always, entirely encapsulated within global homeranges (

n

=

77).We used analysis of covariance (

) to test foreffects of sex, date, year, landscape and tag attachmentmethod (tail mount vs. harness) and their interactionson log-transformed home range areas. Two definitionsof landscape were tested: (i) nesting in arable farmland,pastoral farmland (present only in the mixed studyarea) and gardens; (ii) nesting in gardens, nesting onfarmland within 100 m of a cluster of at least four gar-dens and nesting on farmland more than 100 m fromgarden habitat (both three-level factors). Including thenumber of fixes (and the number of fixes squared) in themodel accounted for the relationship between homerange area and the number of locations.

We used generalized linear mixed models to identifyfactors influencing distance moved from nests duringperiods when adult thrushes were tied to nests, apply-ing the same criteria used to define nesting home ranges(above). Distance data were analysed separately forfixes collected during the egg and chick stages (908 fixesfrom 50 birds) to those collected during the fledglingstage (526 fixes from 39 birds). In addition to the fac-tors tested for home range areas (above), we also testedwhether the amount of rainfall during the preceding7 days influenced distance moved from the nest. Weallowed for non-independence of fix data from the sameindividual bird by declaring bird as a random factor.Square-root transformations were used to normalizethe distance data. Models were fitted using procedures

and

in SAS (SAS Institute Inc. 1999), and astep-up approach to model building was adopted.

Selection of home ranges within the study area

We used compositional analysis (Aebischer, Robertson& Kenward 1993) to describe habitat selection withineach study area. The area of utilized habitat wasdefined by global home ranges (or nesting home rangesfor three of the 86 birds where all fixes related to nestingperiods) and the area of available habitat was providedby previously defined boundaries for each study area(based mainly on land ownership boundaries). Ana-lyses were based on eight habitat categories: woodland(trees generally > 5 m tall); scrub (woody vegetationgenerally < 5 m tall); cereal fields; arable ‘break crops’(oilseed rape, linseed and small areas of maize andgame cover plus, in Essex only, beans and potatoes);grazed grassland; ungrazed grassland (including roughgrass, field margins, road verges and amenity grass aswell as silage and hay); field boundaries; and gardens(mainly domestic gardens but also farm buildings andtarmac roads). Randomization tests were used toassess whether habitat selection was significantly non-random and to determine whether selection differedbetween pairs of habitats (Aebischer, Robertson &Kenward 1993).

Coarse-scale selection of habitats within home ranges

We next considered habitat selection within homeranges at two levels of resolution. A relatively coarse-scale analysis considered variation in fix density acrossthe eight habitat categories defined above. For each ofthese habitats we tabulated the numbers of fix locationsand available area within each global and nesting homerange. Habitat usage data were available for 40 globaland 31 nesting home ranges from the mixed study areaand for 38 global and 23 nesting home ranges from thearable study area. As most nesting period fixes (88% of1185) were within 100 m of the nest, and less than 1%were more than 200 m from the nest, we assumed thatall habitat within home ranges was equally available tothrushes.

We considered factors affecting fix density withinhome ranges using the following log-linear model:

log

e

(

f

ijk

)

=

log

e

(

a

ij

)

+

log

e

(

g

j

)

+

log

e

(

h

i

)

+

log

e

(

k

i

)

×

covariate

k

where

f

ijk

is the number of fixes in habitat

i

, from homerange

j

(and in covariate level

k

), and

a

ij

is the area ofhabitat

i

in home range

j

(for covariate level

k

) declaredas an offset variable that effectively converts the dependentvariable to fix density. The

g

j

are nuisance parametersdescribing differences in the numbers of fixes betweendifferent home ranges. The

h

i

are habitat-specific co-efficients reflecting variation in fix density betweenhabitats relative to a value of 1 for an arbitrary referencehabitat, which in this study was the relatively ubiqui-tous field boundary category. The model was fittedusing procedure

in SAS (SAS Institute Inc.1999) with a log-link function and Poisson error distri-bution. Although the numbers of available fixes variedconsiderably between birds and home ranges (above),use of the log-linear model provided automatic weight-ing according to sample size.

Green, Tyler & Bowden (2000) used log-linearmodels to test for seasonal changes in the pattern of habitatselection, and we extended that approach to consider awide range of potential methodological and ecologicalcovariates. The

k

i

are habitat-specific coefficientsdescribing changes in fix density associated with anycovariate, which in this study included tag attachmentmethod (tail mount vs. back-pack in 1999 only), homerange type (global vs. nesting), bird gender, year (1999vs. 2000), season (early: 19 February

−

16 April; middle:16 April

−

31 May; late: 1 June

−

3 August), nesting land-scape (nest within 100 m of a cluster of at least fourgardens vs. nest further than 100 m from garden habitat)and study area (mixed vs. arable). All of these weremodelled as two-level factors except the three-levelseason covariate, which had to be modelled as a con-tinuous (slope) effect.

The statistical significance of covariates and of vari-ation in fix density across habitats was assessed usingGreen, Tyler & Bowden’s (2000) randomization tests

279

Habitat selection by farmland song thrushes



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41

,275–293

(based on 1000 replicates), in which the individual birdwas the unit of replication, as is desirable in studiesof resource selection (Manly, McDonald & Thomas1993). Each randomization replicate involved ran-domly shuffling habitat labels for each bird, fitting thelog-linear model and using the parameter estimates tocalculate a weighted variance test statistic, which ensuredappropriate influence for abundant but little-used, andrare but heavily used, habitats (Green, Tyler & Bowden2000). The statistical significance of differences in rel-ative preference between pairs of habitats was alsoassessed using Green, Tyler & Bowden’s (2000) ran-domization tests, in which the labels of the pair ofhabitats involved were shuffled randomly for each bird.All randomization tests were programmed using SAS(SAS Institute Inc. 1999).

Fine-scale selection of habitats within home ranges

Given the large number of available radio-fix locationswe attempted to relate variation in fix density to finerscale habitat features. Limitations in fix precision pre-vented the confident assignment of individual fixes tofine-scale habitat features, so we adopted a correla-tional approach in which home ranges were subdividedinto parcels of land positioned, as far as possible, toencompass potentially attractive habitat features suchas field and woodland boundaries, gardens and wet fea-tures such as ponds. We attempted to minimize theamount of such potentially important habitat on theedges of parcels, where fixes were more likely to bewrongly assigned to neighbouring parcels.

A total of 760 parcels was defined for 46 birds in themixed study area, and 586 parcels for 40 birds in thearable study area. Although it was not necessary forparcels to have identical areas or shapes, we aimed foran average parcel area of 0·25 ha, which reflected atrade-off between the resolution with which fine-scalehabitat features were defined and fix precision (above).Actual parcel area averaged 0·28 ha in both studyareas, and 80% of parcels were between 4·6 and 7·5times larger than average fix precision in the mixed areaand between 5·0 and 8·2 times larger in the arable area.To minimize further the incorrect allocation of fixes toneighbouring parcels (due to imprecise fixing), we avoidedlong, thin parcels by ensuring that the ratio of maxi-mum : minimum parcel widths did not exceed 4 : 1.

Having subdivided all home ranges into parcels, werecorded the areas (or presence–absence) of 34 habitats(listed in the Appendix) in each of the 1346 parcels. Wethen tested for relationships between fix density andassociated habitat composition using the following log-linear model:

where

f

ikt

is the number of fix locations in parcel

i

, forbird

k

during season

t

(early

=

0, middle

=

1, late

=

2,see above),

a

i

is the area of the

i

th parcel (declared as anoffset),

bird

k

is a factor with one level for each bird,

hrtype

is a factor specifying home range type (0

=

global home range, 1

=

nesting home range),

dist

is dis-tance of the parcel centre from the nest,

hab

xi

is the pro-portion of the area of parcel

i

that comprises habitat

x

,and

t

is season. The

hrtype

term allowed for overall dif-ferences in fix density between nesting and global homeranges, while the

hrtype

×

dist

and

hrtype

× dist 2 termsallowed for non-linear declines in fix density withincreasing distance from the nest for nesting homeranges only. The four habitat terms allow for non-linearrelationships between fix density and proportional areaof habitat within parcels, the form of which couldchange with season.

We tested for relationships between fix density andhabitat composition separately for each study areaby fitting the above model for each of the 34 habitatvariables (see the Appendix) in turn, and then using ajack-knife procedure to test the statistical significance ofthe habitat (A) that accounted for most variation infix density as reflected in the model with the smallestAkaike’s information criterion (AIC). The jack-knifeprocedure involved excluding the data from each indi-vidual bird in turn and refitting the same model tothe remaining data, thereby making individual birdsthe unit of replication. The parameter estimates for thefour habitat terms were then converted to pseudovaluesand their departure from a mean of zero tested using at-test (Manly 1991). We then refitted the model retain-ing all significant habitat A terms (and non-significantmain effects where quadratic and seasonal terms weresignificant) and including each of the remaining 33habitat variables in turn. Having identified the habitat(B) that accounted for most of the remaining variationin fix density, we then subjected model terms for habi-tats A and B to the jack-knife. This step-up modellingprocess with jack-knife significance testing for allhabitats at each stage of model building continued untilno further significant predictors of fix density could beidentified.

Although intercorrelations between habitat vari-ables often hinder the interpretation of multivariatemodels, this was not a serious problem in this study.Absolute values of Pearson’s correlation coefficientsbetween all pairs of continuous habitats only exceeded0·3 in eight out of 378 cases in the mixed study area, andin five out of 435 cases in the arable study area.

To aid interpretation of the observed patterns of habi-tat selection we assessed soil penetrability and earth-worm biomass in four habitat categories in each of thetwo study areas. Earthworms are probably the singlemost important component of the breeding season diet

log ( ) log ( ) log ( ) log ( ) log ( ) log ( ) log ( ) log ( ) log ( ) log ( )

e e e e

e

e e

e e

e

f a bird hrtypehrtype disthrtype dist habhab hab thab t

ikt i k

h xi

xi xi

xi

= + ++ ×+ × ++ + ×+ ×

2

2

2

280W. J. Peach et al.

© 2004 British Ecological Society, Journal of Applied Ecology, 41,275–293

(Davies & Snow 1965; Gruar, Peach & Taylor 2003),and earthworm biomass is sensitive to soil moisturelevels (Gerard 1967; Green, Tyler & Bowden 2000). Soilsurface earthworm biomass was assessed in arable fields,permanent grass fields (grazed and ungrazed), ditchesand woodlands in both study areas during April andJune 2000. The numbers of locations sampled duringApril 2000 was 24 in the mixed study area and 25 in thearable study area, comprising four and five arable fields,respectively, seven and five permanent grass fields, sixand seven woodland blocks, and seven and eight ditches.Approximately half of these locations were resampledin June 2000 to assess seasonal changes in earthwormavailability. All sampling locations were within homeranges of song thrushes and were considered typical ofthe locality.

We characterized earthworm densities in each loca-tion by using a garden spade to collect 12 surface soilsamples (5 cm deep, 15 × 15 cm area) evenly spacedalong ditches, or along transects diagonally bisectingfields or woodland blocks. Soil samples were collectedbetween 10.00 and 16.00 BST, sealed in plastic bags forup to 48 h after collection, and hand sorted with the aidof running water and 2-mm mesh sieves. All earth-worms were cleaned, blotted dry, counted and weighed(total sample wet weight) to the nearest 0·1 g. Althoughsurface soil samples will have under-recorded largerindividuals of some deep-burrowing species likeLumbricus terrestris (L.) (Gerard 1967), non-burrowing,diurnally active species are likely to constitute themajority of the song thrush diet (Török 1985). Analysisof variance was used to investigate variation in earth-worm biomass between study areas, habitats and months,and associated interactions. Separate analyses wereperformed for each of the four habitats. A square-roottransformation was needed to normalize the earthwormbiomass means for each location.

Soil surface penetrability is an indirect measure ofsoil moisture (Green 1988) and therefore a potentialpredictor of earthworm availability. We assessed soilpenetrability six to eight times between March and July1999 at each of 74 different locations covering fivemajor habitats (nine arable fields, 13 grass fields, 21ditches, eight woods and 23 gardens split evenly betweenthe study areas), and twice during 2000 (April andJune) at all 49 earthworm-sampling locations (above).We measured the force required to insert a 5-cm steelrod (4-mm diameter) into the soil surface. The steelprobe was attached to a wooden dowel mounted withina plastic pipe and connected via a wire cradle to a 20-kgspring balance (following Green 1988). On each sam-pling occasion in each location, we measured soilpenetrability at 20–24 positions spread evenly along thebase of the ditch or along a transect diagonally bisect-ing the field, woodland block or garden habitat.

Analysis of covariance was used to test for seasonaltrends, differences between study areas and effects ofrecent rainfall (during the 7 days preceding sampling)on soil penetrability (log-transformed location means).

Location was included as a factor in these s,thus allowing us to investigate variables affecting ratesof change in soil penetrability across sampling locations.Daily rainfall statistics were obtained from weatherstations located 3 km north of the respective study areas.Separate s were conducted for each of the fivehabitat categories.

Results

Global home ranges (GHR) were significantly largerthan nesting home ranges (NHR) after allowing forvariation in the numbers of fixes contributing to eachhome range (, F1,28 = 15·8, P < 0·0001; havingmean areas of 3·3 and 1·1 ha, respectively). The averageareas of NHR and GHR did not differ between studyareas. NHR were larger for males than for females (rawmean areas were 1·5 and 0·5 ha, respectively), increasedthrough the breeding season (Fig. 1), were larger in1999 than in 2000, and were larger on farmland whengarden habitat was within 100 m of the nest (Table 1).The effects of season, year and landscape remained sig-nificant in analyses of the male NHR, but were notsignificant in analyses restricted to female NHR. Meanmale NHR were 2·9 ha for farmland nests within 100 mof at least four gardens, 1·0 ha for farmland nests with-out four gardens within 100 m and 1·4 ha for nests ingardens. Proximity of gardens to farmland nests influ-enced the area of GHR in a similar manner (Table 1;mean GHR were 5·4 ha for farmland territories within100 m of gardens, compared with 3·0 ha for farmlandterritories further than 100 m from gardens and 2·3 hafor territories centred on gardens).

During the incubation and chick stages, male thrushesmoved further away from nests than females but therewas no seasonal trend or difference between sites inaverage distances moved (Table 2). Rainfall during the

Fig. 1. Seasonal changes in the area of nesting home ranges(minimum convex polygons) for male and female adult songthrushes. Areas are plotted against the median date of allreliable fixes for each bird (where 1 = 1 March).

281Habitat selection by farmland song thrushes


preceding week was associated with shorter move-ments away from nests during both the nest and fledg-ling stages (Table 2). The smaller NHR in 2000 (above)probably reflect the much greater rainfall in both studyareas during that summer (89% more rainfall in Sussexduring April–July 2000 and 44% more in Essex). Therewas a pronounced seasonal increase in the distancesmoved by males (but not females) during the fledglingstage.

Territorial settlement involved significantly non-random habitat selection in both study areas (mixed areaWilk’s Λ = 0·154, randomization P < 0·001; arable areaWilk’s Λ = 0·121, randomization P < 0·001). Fieldboundaries and woodland were selected in the mixedstudy area, while all arable cultivation was avoided(Table 3). Field boundaries and gardens were selectedin the arable study area, while woodland and arablebreak crops were avoided. Woodland was selected inthe mixed study area but avoided in the arable area,while cereal was strongly avoided in the mixed area butnot in the arable area.

-

Habitat utilization differed markedly between studyareas (χ2 = 920·6, 7 d.f., P < 0·001), with a much higherproportion of fixes in the mixed study area being recordedin woodland (32·2% compared with 2·2% in the arablearea) and grazed grass (11·2% vs. 3·6%), and a muchlower proportion in arable crops (6·3% vs. 23·9%) andgardens (15·7% vs. 33·8%) (Fig. 2a). Combined usageof woodland and all grassland (grazed, ungrazed andrank) accounted for 53% of fixes in the mixed studyarea and only 13% in the arable area. Field boundariesaccounted for 23% of fixes in both study areas.

The pattern of fix density across the eight habitatcategories did not differ significantly between tagattachment methods (randomization significance levelsfor mixed and arable areas: P = 0·20 and 0·41, respectively),home range types (P = 0·79 and 0·58), sexes (P = 0·96and 0·83), years (P = 0·23 and 0·70) or seasons (P = 0·67and 0·98). There was some indication that fix density inthe arable study area (although not in the mixed area)differed between home ranges centred on gardens andon farmland (P = 0·076), with garden-nesting thrushesmaking greater use of gardens. Relative fix density did

Table 1. Factors influencing areas of (a) nesting and (b) global home ranges (summary statistics from models). Statisticalsignificance (P) of effects in the final model and changes in model strength (R2) are listed

Independent variables P R2 (%) Direction of effects

(a) Nesting home rangesNumber of fixes < 0·0001 22·6 PositiveSex < 0·0001 40·8 Males > femalesLandscape 0·004 50·7 A > C > B*Year 0·0138 56·0 1999 > 2000Date 0·0074 62·3 Positive (Fig. 1)

(b) Global home rangesNumber of fixes 0·0012 18·7 Positive(Number of fixes)2 0·0352 25·7 NegativeLandscape 0·0032 36·7 A > B > C*

*Landscape codes: A, nest on farmland and within 100 m of at least four separate gardens; B, nest on farmland and fewer than four separate gardens within 100 m; C, nest in garden.

Table 2. Factors affecting distances moved by parent song thrushes from nests during (a) the egg and chick stage (n = 908 fixesfrom 50 birds) and (b) while adults were caring for recently fledged young (n = 526 fixes from 39 birds). Statistical significance (P)of effects in the final model and changes in model parsimony (AIC) are listed

Independent variables P AIC Direction of effects

(a) Egg and chick stageRainfall during previous 7 days 0·0054 4118·4 NegativeSex 0·0007 4108·0 Males > females

(b) Dependent fledglings stageDate 0·0003 2542·4 PositiveRainfall during previous 7 days 0·0045 2541·1 NegativeSex 0·0656 2539·5 Females > malesSex × date 0·0240 2540·8 Males > females



not differ between study areas either when all data wereanalysed together (P = 0·28) or in separate analyses ofhome ranges centred on gardens (P = 0·86) or on farm-land (P = 0·34).

Fix density differed significantly from random inboth the mixed and arable study areas (P = 0·005and 0·009, respectively). In both areas, fix densities werehighest in scrub and lowest in cereal fields (Fig. 2b).Pairwise significance tests indicated that fix densityin the mixed study area was significantly lower incereals, arable break crops and grazed grass than inother habitats (Table 4). In the arable study area, fix

density was significantly higher in scrub and fieldboundaries, and significantly lower in cereal and grazedgrass fields, than in other habitats. Although the overallpattern of habitat selection did not differ betweenstudy areas (above), the relative intensity of usage ofarable break crops was greater in the arable study area,while the relative intensity of usage of woodland andungrazed grassland was greater in the mixed study area(Fig. 2b).

The difference between the observed numbers of fixesin each habitat and the numbers expected if habitatusage was random (Fig. 2c), is a means of integrating

Table 3. Pairwise habitat preferences reflecting habitat selection at the level of the study area (home ranges within study areas)as shown by compositional analysis. Signs indicate preferences for habitats down the left-side column relative to those runningalong the top row. Three +/− symbols represent statistically significant departures from random (at P < 0·05) while single symbolsreflect non-significant preferences. Thus field boundaries were preferred to woodland in both study areas, but the preference wasonly statistically significant in the arable area. Ranks indicate the most preferred (1) and most avoided (8) habitats in each studyarea. Habitats are defined in the Methods

WOO GAR SCR GRA UNG OTH CER Rank

Field boundary (BOU) Mixed + + + + + + + + + + + + + + + + + + + 1Arable + + + + + + + + + + + + + + + + + + + 1

Woodland (WOO) Mixed + + + + + + + + + + + + + + + + + + 2Arable − − − − − − − − − − + − − − 7

Gardens (GAR) Mixed + + + + + + + + + 3Arable + + + + + + + + + + + + + + + 2

Scrub (SCR) Mixed + + + + + + + + 4Arable + + + + + − 4

Grazed (GRA) Mixed + + + + + + + 5Arable − + − 6

Ungrazed (UNG) Mixed + + + + + + 6Arable + − 5

Other arable (OTH) Mixed + 7Arable − − − 8

Cereal (CER) Mixed 8Arable 3

Table 4. Pairwise habitat preferences reflecting habitat selection within home ranges as shown by generalized linear modelling offix density with randomization significance tests. Signs indicate preferences for habitats down the left-side column relative to thoserunning along the top row. Three +/− symbols represent statistically significant departures from random (at P < 0·05) while singlesymbols reflect non-significant preferences. Ranks indicate the most preferred (1) and most avoided (8) habitats in each study area.Habitats are defined in the Methods. There was no difference (ND) in fix density between gardens and other arable habitats in thearable study area

GAR UNG WOO BOU GRA OTH CER Rank

Scrub (SCR) Mixed + + + + + + + + + 1Arable + + + + + + + + + + + + + 1

Garden (GAR) Mixed + + + + + + + + + + 2Arable + + − + ND + + + 3·5

Ungrazed (UNG) Mixed + + + + + + + 3Arable − − − − + − + 6

Woodland (WOO) Mixed + + + + + + + + 4Arable − + − + 5

Field boundary (BOU) Mixed + + + + + + + 5Arable + + + + + + + 2

Grazed (GRA) Mixed + + 6Arable − + 7

Other arable (OTH) Mixed + 7Arable + + + 3·5

Cereal (CER) Mixed 8Arable 8



intensity of usage and habitat availability. This pres-entation of the data emphasizes the importance of fieldboundaries and gardens in both study areas, and ofwoodland in the mixed study area only. Cereals werestrongly avoided in both study areas (Fig. 2c). Arablebreak crops and grazed grassland were avoided in themixed study area but not in the arable area. Althoughthe density of fixes in scrub was high in both studyareas, the importance of this habitat was limited by itsscarce availability.

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Habitat correlates of fix density were similar for the twostudy areas. A combination of woodland edge, wetditches and bare ground in gardens accounted for 81%and 71% of the total AIC reduction for the mixed andarable study areas, respectively (Table 5). By far the

strongest predictors of fix density were woodland edgein the mixed study area (accounting for 55% of totalAIC reduction) and wet ditches with hedges (38% ofAIC reduction) in the arable study area. The correlationbetween the areas of woodland edge and wet ditcheswithout hedges in the mixed study area (r = 0·35) sug-gested that one or both habitats may have been import-ant to foraging thrushes. Other positive correlates offix density were unmown green cover (both study areas)and bare ground in gardens (both study areas) andoutside gardens (arable study area). The correlationsbetween area of shrubs and hedges and area of bareground in gardens (Table 5) reflected a tendency forthese habitats to occur together in garden borders, andin our experience song thrushes in this habitat wereusually foraging on bare soil. Fix density in the arablestudy area was also correlated with the area of scrub,particularly early in the breeding season (Fig. 4d).

Fig. 2. Habitat utilization and selection by adult song thrushes in the mixed (shaded) and arable (stippled) study areas.Habitat utilization (a) is the total number of radio fixes in each habitat category, intensity of habitat usage (b) presents indices offix density (derived from a log-linear model) relative to an arbitrary fix density of 1 in field boundaries, and habitat utilizationallowing for availability (c) is the difference between the observed numbers of fixes in each habitat and the number expected if fixdensity was equal across all habitats.



All of the significant relationships between fixdensity and habitat variables involved either a positiverelationship or a non-linear relationship in which fixdensity initially increased with habitat area and thendeclined beyond some turning point (Figs 3 and 4).The latter may reflect a preference for habitat edges andavoidance of areas far from cover. There was only lim-ited evidence for seasonal changes in the relationshipsbetween fix density and habitat composition (Table 5),and this involved relatively intensive early season usageof land parcels containing woodland edge (both areas),scrub (arable study area) or bare ground in gardens(mixed study area) (Figs 3 and 4).

With the exception of grazed grass fields in the mixedstudy area, earthworm biomass during April 2000 wassimilar across habitat types and between study areas(Fig. 5). There was a pronounced seasonal decline inearthworm availability in all habitats in both study areas(P < 0·001; Fig. 5). The decline varied between habitats

(P < 0·05) and was most pronounced in grass and arablefields and least pronounced in woods. By June, earthwormswere scarce in all habitats in the arable study area butwere still relatively abundant in the woodland and grassfields of the mixed study area (Fig. 5). Separate habitat-specific analyses indicated significant seasonal declinesin earthworm biomass in all habitats and significantlygreater earthworm biomass in the grass fields andwoodlands of the mixed study area (Table 6a).

Earthworm biomass in 2000 was negatively relatedto soil penetrability in all four habitats (Fig. 6 andTable 6b), although the form of the relationship differedsignificantly between habitats (but not study areas).Harder surface soil conditions (reduced penetrability)accounted for some, but not all, of the seasonal declinein earthworm biomass, and for the lower earthwormbiomass in grass fields during April and June in thearable study area (Table 6b and Fig. 6c). Soil penetrabilitywas similar in the woodlands of the two study areas andcould not account for the higher earthworm densitiesin the mixed area (Table 6b and Fig. 6d). Surface soilsin the arable study area were significantly harder on

Table 5. Habitat predictors of fine-scale spatial variation in fix density

Habitat variables Relationship form and jack-knife significance*

Relative strength of habitat variables(reduction in AIC) Correlated habitats†

Mixed study areaDry woodland edge LIN + + +, QUAD − − −, LIN × SEAS − −,

QUAD × SEAS + + +322·7 Wet ditch, no hedge

(0·35)Unmown green cover LIN + + + 100·4Bare ground in garden‡ LIN + + +, QUAD − −, LIN × SEAS (−),

QUAD × SEAS +92·6 Garden shrubs and

hedges (0·63); Lawns/amenity grass (0·33)

Wet ditch plus hedge LIN + + +, QUAD − 61·8Compost heap PRES + 11·4

Arable study areaWet ditch plus hedge§ LIN + + +, QUAD − 208·1 Unmown green

cover (0·30)Bare ground LIN + + +, QUAD − − − 80·6Bare ground in garden LIN +, QUAD − 75·6 Garden shrubs and

hedges (0·47)Scrub LIN +, LIN × SEAS − 75·2Dry woodland edge LIN (+), QUAD (−), LIN × SEAS − −,

QUAD × SEAS +59·4

Wet ditch, no hedge LIN + + + 44·0

*Relationships between fix density and proportional area of habitat: LIN, linear coefficient; QUAD, quadratic coefficient; SEAS, season modifier; PRES, presence of habitat factor. Signs of parameters and statistical significance: +++/− − −P < 0·005;++/− −P < 0·01; +/−P < 0·05; (+/−)P > 0·05.†Habitats are listed if the absolute Pearson’s correlation coefficient (quoted in parentheses) with the model habitat variable exceeded 0·3.‡The area of ‘shrubs and hedges in gardens’ entered the model before the area of ‘bare ground in gardens’ but became non-significant in the presence of the latter. These two garden variables were by far the most strongly intercorrelated of all the mixed study area habitat variables (r = 0·63, P < 0·001), suggesting that the study has limited power to differentiate between the possible effects of these habitats on fix density.§The first habitat variable to enter the arable study area model was ‘cereals’ but this became non-significant in the presence of ‘wet ditch plus hedge’. Fix density showed a significant quadratic relationship with cereal area, the main effect being positive but only weakly significant (P < 0·09) and the quadratic term being negative and highly significant (P < 0·003). The form of the relationship is shown in Fig. 4g.



grass fields (April and June) and arable fields (Juneonly) than in the mixed study area (Fig. 6).

Soil penetrability in 1999 was initially similar acrosshabitats and study sites (Fig. 7). In all five habitatsmonitored, soil hardness increased significantly (P < 0·005)over the course of the breeding season, and the rate atwhich the soil hardened was significantly greater (P <0·005) in the arable study area than in the mixed studyarea. Recent rainfall also significantly softened the soilin all habitats (P < 0·005 for ditches, arable fields andgardens, P < 0·05 for woods and grass fields). Thefaster rate of hardening of the surface soils in the arablestudy area was not a consequence of less rainfall in that

area during 1999; on the contrary, total rainfall for themonths March–July was greater in the arable studyarea (235 mm) than in the mixed study area (193 mm).

Discussion

Selection of habitats at the territory settlement scaleemphasizes the importance of nesting cover for songthrushes. Field boundaries, woodland and gardens pro-vided most nesting sites in the mixed study area and werepreferred over other habitats (Table 3). Field boundaries

Fig. 3. Relationships between fix density and habitat composition within home ranges in the mixed study area (see Table 5 formodel details). Plotted points are mean numbers of fixes (filled dots with standard errors) for bins of at least 20 land parcels havingsimilar proportions of the habitat being considered. For habitats where relationships varied seasonally (Table 5), mean numbersof early (diamonds), middle (squares) and late (triangles) season fixes are also presented.

Table 6. Factors affecting soil surface earthworm biomass in four habitats in each study area during April and June 2000.Analyses of covariance were conducted with (a) and without (b) soil surface penetrability included as a covariate. Significancelevels: ***P < 0·005, **P < 0·01, *P < 0·05, NS not significant

Independent variables (direction of effects)

Habitats (number of fields/locations)

Woodland (20) Ditch (22) Grazed grass (18) Arable (14)

(a) Analyses excluding penetrabilityMonth (April > June) *** *** *** ***Study area (mixed > arable) * NS *** NSMonth × study area NS NS NS NS

(b) Analyses including penetrabilityPenetrability (negative) *** *** *** NS†Month (April > June) * * * ***Study area * NS NS NSMonth × study area NS NS NS NS

†The effects of penetrability and month on earthworm biomass in arable fields were confounded. Penetrability was a highly significant predictor of earthworm biomass in the absence of the month factor.



and gardens also provided most nesting sites in thearable study area and, in contrast to the mixed study area,the relatively small areas of woodland were avoided.Cereal fields were strongly avoided as foraging habitats(Table 4 and Figs 2 and 4); the relatively high rankingof cereal fields during territory settlement in the arablestudy area was probably an artefact of most fieldboundaries lying adjacent to cereals. The importance ofgardens and woodland as predictors of breeding distri-bution on lowland farmland has been highlightedpreviously (Mason 1998, 2000).

While the intensity of habitat usage within homeranges was broadly similar in the two study areas, there

were large differences in habitat utilization. Fieldboundaries and gardens were heavily utilized in bothstudy areas, but woodland and grassland accounted for53% of fixes in the mixed study area compared with just13% in the arable study area. Grassland was also heav-ily utilized by foraging song thrushes during Januaryand February (Peach et al. 2002). Permanent grasslandand woodland are likely to constitute relatively food-rich habitats for song thrushes (Fig. 5; Tucker 1992),being less affected by ploughing, drainage and pesti-cide application than arable fields. In addition to thegreater availability of woodland and permanent grass-land in the mixed study area, there was also evidence

Fig. 4. Relationships between fix density and habitat composition within home ranges in the arable study area (see Table 5 formodel details). Plotted points are mean numbers of fixes (filled dots with standard errors) for bins of at least 20 land parcels havingsimilar proportions of the habitat being considered. For habitats where relationships varied seasonally (Table 5), mean numbersof early (diamonds), middle (squares) and late (triangles) season fixes are also presented. Although the area of cereals was notretained in the final model (Table 5), it was the strongest single predictor of fix density (Table 5§).



that earthworm availability in these habitats may havebeen lower in the arable study area (Fig. 5), particularlylater in the breeding season, as a consequence of driersoil conditions (Figs 6 and 7). The persistence ofdamper soil conditions later in the breeding season inthe mixed study area is likely to have increased the gen-eral availability of earthworms and other soil inverte-brates relative to that in the arable study area, and tohave had beneficial effects on chick and adult bodycondition (Gruar, Peach & Taylor 2003). This studytherefore provides strong support for both of our start-ing hypotheses (see the Introduction): the availabilityand the quality (prey availability) of key foraging hab-

itats (woodland and grassland) were lower in the arablelandscape, and seasonal declines in the availability ofkey prey resources (earthworms) were more pro-nounced in the arable landscape. It is plausible that thelower renesting propensity among thrushes in theEssex study area (see the Introduction) was a directconsequence of relatively severe food shortages in thatarable landscape.

As differences in rainfall could not account for thepersistence of softer soil conditions in the mixed studyarea in 1999, it is pertinent to consider what other fac-tors might be involved. Soil type influences the amountof water retained in surface soils. The arable study area

Fig. 5. Mean earthworm biomass (± SE) in surface soils in four habitats in each study area during April and June 2000. The top5 cm of soil was sampled.

Fig. 6. Relationships between mean soil surface earthworm biomass and soil penetrability in four habitats sampled in the mixed(open squares) and arable (filled circles) study areas during April and June 2000. Each point represents average biomass in the top5 cm of soil in a single field, wood or ditch sampled during either April or June (some were sampled in both months).



was dominated by heavy clay soils while the mixedstudy area included fine loamy sandstone and heavyclay in approximately equal proportions. Under dampconditions fine loamy soils tend to hold more moisturethan clay soils, but clay soils retain moisture better asconditions dry out, so the relative moisture contents ofthese two soil types is likely to vary with prevailing con-ditions (Ward & Robinson 1989). The greater extent ofunder-field drainage in the arable study area is prob-ably more likely to account for the drier mid-summersoil conditions than soil texture. Most of the farmlandin the arable study area had under-field drainage pipesbut most fields (mainly permanent pasture) along thebase of a river valley in the mixed study area lackedunder-field drainage and often remained damp intoMay and June. Essex was one of the most heavilydrained counties in England and Wales during the1970s, while west Sussex experienced much lower ratesof agricultural drainage (Robinson & Armstrong1988). Differences in topography between study areasmight have resulted in greater natural drainage andrun-off of water in the arable area, which was centredon Toot Hill and surrounded by lower lying land. Themixed study area lay at the base of the South Downsand was surrounded by higher ground.

In both study areas most woodland song thrush ter-ritories were centred on woodland edge rather than

woodland interior (Fuller & Whittington 1987; Mason2001), and dry woodland edge was the strongest pre-dictor of fix density in the mixed study area and aweaker predictor in the arable study area (Table 5).This apparent preference for woodland edge mightreflect the utilization of adjacent foraging habitats (suchas grassland or ditches) and the higher shrub coveralong woodland edge (Fuller & Whittington 1987).The latter was evident in fine-scale habitat descriptionsof home ranges in the mixed study area, where theproportions of woodland edge with > 50%, 10–50% and< 10% shrub cover were 34%, 40% and 26%, respec-tively, compared with 28%, 25% and 47% in woodlandinterior. Although woodland accounted for a muchhigher proportion of fixes in the mixed study area, thiswas mainly a consequence of the greater availability ofwoods and only partly due to a slightly greater intensityof usage (Fig. 2). The much stronger preference forwoodland during territory settlement in the mixedstudy area (Table 3) suggests that the woodland therewas more attractive to thrushes, at least as a nestinghabitat. Although there were larger woods within andon the edge of the mixed study area, average woodlandarea did not differ significantly between study areas[median (range) woodland areas were 0·44 ha (0·1–103·6 ha) in the mixed area and 0·42 ha (0·1–5·0 ha) inthe arable area; Mann–Whitney P > 0·8]. Woodland

Fig. 7. Seasonal changes in mean soil surface penetrability in five habitats sampled in the mixed (open circles) and arable (filledcircles) study areas during 1999. Mean penetrability for each field, ditch, wood or garden habitat is plotted against the date ofsampling (where 1 = 1 March).



edge within home ranges had proportionately morescrub cover in the mixed study area (percentages ofwoodland edge with > 50%, 10–50% and < 10% scrubcover were 34%, 40% and 26%, respectively, in the mixedstudy area, compared with 28%, 32% and 39% in thearable area), and this may have increased its suitabilityas nesting habitat. Woodlands with perimeter hedgesare preferred by breeding song thrushes in arable land-scapes (Hinsley et al. 1995).

Gardens were preferred nesting and foraging habi-tats in both study areas (Tables 3 and 4), and the largerhome ranges (and greater distances moved) of thrushesnesting on farmland within 100 m of garden habitat(Tables 1 and 2) suggests that provisioning thrusheswere prepared to travel relatively large distances to visitgardens. The selection in both study areas of bare groundin gardens (Table 5) reflects an apparent preference fordisturbed soil as a foraging habitat, and raises the pos-sibility that song thrushes might be exposed to mollus-cicide pellets, which can be fatal to birds when ingested(Pesticides Safety Directorate 1996). The greater usageof gardens and arable fields in the arable study area(Fig. 2a) may reflect a general lack of alternative higherquality foraging habitats such as woodland and grass-land. Availability of earthworms in arable fields is likelyto have been low once surface soils dried out (Fig. 5),but other invertebrate prey like Coleoptera, Lepid-optera and Mollusca were available in oilseed rape, beansand cereals. Field boundaries were important foragingand nesting habitats in both study areas, particularlythose that remained damp throughout the summer andthat provided hedges as cover (Table 5). Rank, ungrazedgrassland was also a favoured foraging habitat, par-ticularly in the mixed study area (Table 5), where songthrushes were often seen collecting snails from longgrass.

There was little evidence of seasonal changes in thepattern of habitat selection. This suggests that largeseasonal declines in the availability of earthwormsoccurred in most habitats and swamped any subtlerseasonal changes in the relative quality of differenthabitats. Although failing to achieve statistical signi-ficance, there was some evidence from the arable studyarea of increasing usage of arable break crops throughthe breeding season. On several occasions during Juneand July, the same non-breeding adult thrushes werelocated in the same oilseed rape or bean fields for sev-eral days in succession. The negative influence of recentrainfall on average distances moved by provisioningadult thrushes (Table 2) probably reflects short-termchanges in soil dampness and the availability of keyinvertebrate prey (i.e. drier soils force provisioning thrushesto travel further from the nest in search of food). Theseasonal increase in the movements of males (but notfemales) with dependent fledglings (Fig. 1 and Table 2)might reflect drier late summer soil conditions and pos-sibly late summer separation of broods, in which femalesremain in nesting home ranges and males move away(Hill 1998).

It is plausible that the much higher density of songthrushes in the mixed study area (see the Methods) is aconsequence of the greater availability of nesting cover(especially woodland) and key foraging habitats (espe-cially woodland and permanent grassland). The morerapid drying of surface soils in the arable landscape willhave caused a larger and faster seasonal decline in theavailability of earthworm prey than in the mixed studyarea. Earthworms constituted a higher proportion ofsong thrush diet in the mixed study area, althoughthere was a large seasonal decline in the consumptionof earthworms in both populations (Gruar, Peach &Taylor 2003). Furthermore, body condition of adultthrushes (in both study areas) and chicks (in the arablestudy area only) declined as surface soils became drier(Gruar, Peach & Taylor 2003). Thus, the fewer late summernesting attempts made by thrushes in the arable studyarea (Thomson & Cotton 2000) might have been a directconsequence of food shortages caused by relatively drysoil conditions during May–July

Because our arable study area is typical of muchlowland arable farmland in Britain, it is appropriate toconsider whether losses or degradation of key summerforaging or nesting habitats might have contributed tothe wide-scale declines of song thrushes on lowlandfarmland that occurred between the mid-1970s and themid-1990s (Baillie et al. 2001). Important habitatsidentified in this study were field boundaries (especiallywhen damp and with hedge cover), woodland edge,scrub, grassland (grazed and unmanaged) and bareground in gardens, while cereals and, in the mixedstudy, other arable crops were avoided.

Approximately half of Britain’s hedges were removedbetween 1947 and 1990, with high losses continuingbetween 1978 and 1990 (Barr & Parr 1994). Densitiesof many farmland birds (including song thrushes) areknown to be sensitive to the density of hedgerows (Lack1992), and the loss of hedgerows probably reducedthe carrying capacity of much lowland farmland forsong thrushes. Widespread conversion of permanentpasture to cereal production, particularly in easternBritain during the 1960s and 1970s (O’Connor & Shrubb1986), amounts to the replacement of a heavily utilized,food-rich resource with a habitat that is stronglyavoided by foraging song thrushes (Table 4 and Fig. 4).Permanent grassland is a relatively rich source ofearthworms (Fig. 5; Tucker 1992) and, although notstrongly selected in this study, it was heavily utilizedby song thrushes during summer (Fig. 2) and winter(Tucker 1992; Peach et al. 2002). Grazing by cows isassociated with increased usage of grass fields by songthrushes in winter (D. Buckingham, unpublished data)and the loss of cattle-grazed pastures from many east-ern regions of Britain has probably removed large areas



of food-rich foraging habitat for birds like song thrushes.The loss of livestock and organic fertilizers from manyarable farms is likely to have reduced the organic con-tent of cultivated soils and their suitability for earth-worms and other soil invertebrates (Edwards 1984).

Song thrushes in the UK probably benefited from a56% increase in total woodland area between 1965 and2001 and an 88% increase in farm woodlands between1981 and 2001 (Forestry Commission 2002). However,the removal of understorey vegetation by increasingdeer populations may have reduced the suitability ofmany lowland woods as nesting and foraging habitatsfor song thrushes (Fuller 2001). Scrub was a relativelyrare but intensively used habitat in both of our studyareas and may be an important factor influencing dis-tribution on arable farmland (Mason 2000). Approxi-mately 60% of scrub was lost in Great Britain between1965 and 1980 (Locke 1987), representing a substantialloss of potential nesting cover and foraging habitat forsong thrushes.

Another aspect of agricultural management thatis likely to have negatively impacted song thrushes isthe widespread installation of under-field drainagesystems. Pipes are installed beneath the soil surface topromote drying of surface soils, and this increasescrop yields, reduces crop disease problems and extendsthe periods when heavy machinery can be used. En-couraged by the provision of government grants andfree technical advice, rates of land drainage in Englandand Wales increased in every decade after the 1940s,peaking during the late 1970s and declining thereafter(Robinson & Armstrong 1988). Approximately half of allland drainage during the twentieth century occurredduring the 1970s, with most of this being concentratedin eastern arable areas of England, especially on claysoils (Robinson & Armstrong 1988). Land drainagewill result in lower densities of soil invertebrates likeearthworms (Fig. 6) and drier surface soils have beenshown to shorten the length of the breeding season ofsnipe Gallinago gallinago (L.) (Green 1988). It seemsplausible that land drainage could have affected songthrushes in a similar manner, and it is notable that thetiming and spatial extent of land drainage in Britainbroadly correspond to that of the song thrush popula-tion decline (i.e. the steepest population decline duringthe period 1975–79 and the worse affected areas beingthe arable-dominated counties of eastern England). Adrier agricultural landscape in lowland Britain hasprobably affected a wide range of birds and other wild-life, particularly declining species such as lapwingVanellus vanellus (L.), starling Sturnus vulgaris (L.) andmistle thrush Turdus viscivorus (L.) that are dependenton soil invertebrates for food.

Our findings indicate that breeding song thrushes requiredense woody vegetation for nesting cover situated close

to damp soils providing soil invertebrates. The smallhome ranges of nesting thrushes emphasizes the needfor nesting and foraging habitats to be close together.Current British agri-environment schemes offer rela-tively few prescriptions that might benefit song thrusheson lowland farmland. Planting and restoration ofhedgerows under the Countryside Stewardship Scheme(CSS) should provide nesting and feeding sites, particu-larly when coupled with sympathetic field marginmanagement. Song thrushes prefer tall, wide, species-richhedges with trees, ditches and adjacent grassland(Hinsley & Bellamy 2000). Of the new ‘arable options’recently incorporated into CSS, song thrushes are mostlikely to benefit from ‘wild bird seed mixes’ incorporat-ing leafy brassicas such as kale (Henderson & Vickery2001).

The various CSS grassland management andcreation measures are unlikely to encourage arablefarmers to take on grazing livestock. Organic farmingmight be expected to provide benefits for song thrushesin arable landscapes because it often involves a returnto mixed farming with livestock, because higher levelsof organic matter in the soil should promote keyinvertebrate prey like earthworms, and because fieldboundaries on organic farms may be managed moresympathetically for birds. There is some evidence thatdensities of song thrushes are higher on organic farmsand this does not appear to be entirely due to thepresence of taller, wider hedgerows (Chamberlain &Wilson 2000).

New farm woodlands created under the Farm Wood-land Premium Scheme and the Woodland Grant Schemehave been shown to be attractive to song thrushes dur-ing summer and winter, especially where the adjoininghedges are dense and contain trees (Vanhinsbergh et al.2002), and their utility might be increased by theencouragement of perimeter shrub cover (Hinsley et al.1995). There are currently no financial incentives forthe planting or restoration of scrub on British farm-land, despite the importance of this habitat to songthrushes and other birds.

New policy initiatives are needed to encourage mixedfarming, damper soil conditions and small, uncroppedfeatures across significant areas of UK lowland farm-land. Given the importance of wet ditches to songthrushes and several other priority farmland birds (e.g.reed bunting Emberiza schoeniclus L.), considerationshould be given to agri-environment payments for thecreation and maintenance of ditches that remain dampall year round, preferably with hedge cover or close toscrub. Agri-environment payments for raised waterlevels on grass fields have traditionally been aimed atencouraging breeding waders but damp grass fields arelikely to benefit a wide range of birds that feed on soilinvertebrates. Increasingly dry summers in southernand eastern England, as predicted under some globalclimate change scenarios (Hulme, Turnpenny & Jenkins2002), might be expected to reduce the availability ofsoil invertebrates for song thrushes.



Acknowledgements

We are grateful to the farmers, landowners and resi-dents of our study areas in Sussex and Essex whokindly provided access to their land, logistical supportand refreshment. Andy Mitchell and Steve Babbshelped collect data in Essex. Rhys Green advised ondata analysis and Ellen Kelly provided much help withGIS. The compositional analyses were performedusing an Excel macro written by Dr P.G. Smith. TheBritish Trust for Ornithology provided CBC data forthe two study sites. This work was funded by RSPBwith support from Frizzell Insurance.

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Received 11 July 2003; final copy received 4 December 2003



Appendix Habitat categories used to describe the fine-scale composition of home ranges

Habitat categories

Combined area (ha) and (in parentheses) number of parcels containing each habitat

Sussex (760 parcels) Essex (586 parcels)

Fields Permanent pasture grazed by cattle and sheep 26·8 (159) 0 (0)Permanent pasture grazed by horses 15·3 (113) 8·2 (71)Mown grass (silage, hay) 17·6 (98) 10·1 (86)Unmanaged or neglected grass (includes set-aside) 4·3 (52) 8·0 (55)Cereals (mainly winter wheat) 46·1 (213) 46·8 (270)Oilseed rape 0 (0) 11·7 (53)Potatoes 0 (0) 3·4 (17)Beans 0 (0) 14·8 (77)Other arable (mainly linseed, some game cover) 24·2 (163) 3·1 (25)

Field boundaries* Wet ditch plus hedge 2·2 (107) 3·6 (153)Wet ditch, no hedge 1·2 (116) 0·1 (23)Dry ditch plus hedge 0·4 (24) 0·6 (27)Dry ditch, no hedge 0·4 (20) < 0·1 (3)No ditch plus hedge 0·5 (48) 1·2 (80)No ditch, no hedge 0·2 (40) 0·3 (31)

Woodland† Dry woodland interior 24·8 (136) 0·2 (3)Damp woodland interior 0·6 (53) 0·1 (2)Dry woodland edge 15·6 (205) 4·2 (102)Damp woodland edge 1·1 (81) 1·5 (37)

Scrub‡ Scrub 2·2 (53) 1·3 (52)Open ground Bare ground§ (< 10% green cover) 0·2 (35) 0·2 (25)

Unmown green cover¶ (> 10%) 8·0 (279) 11·0 (304)Mown green cover** (> 10%) 1·9 (58) 2·7 (90)

Gardens Bare ground (< 10% green cover) 0·7 (57) 1·1 (106)Unmown green cover (> 10%) 0·6 (34) 1·1 (34)Lawns and amenity grass†† 6·8 (78) 10·7 (164)Shrubs and hedges 1·1 (67) 2·3 (129)Buildings 1·3 (46) 1·4 (38)Pond/wetland feature 0·1 (13) 0·5 (32)Paved/tarmac 3·5 (97) 7·4 (182)P/a bird feeder‡‡ − (25) − (18)P/a compost heap‡‡ − (44) − (19)Number of mature trees − (47) − (69)

Water Ponds, streams, rivers 0·2 (17) 2·0 (37)

*Wet ditches were those containing water or wet mud during April and May.†Woodland ‘edge’ refers to outer 10 m. Damp features in woodland include water-filled or muddy ditches, streams and ponds.‡Scrub is woody vegetation mostly less than 5 m tall (woodland was dominated by trees > 5 m tall).§Includes leaf litter.¶Mainly rank grass and tall herbaceous cover; includes most arable field margins.**Mainly grass verges and footpaths; excludes garden lawns and amenity grass.††Amenity grass was always on the edge of villages.‡‡Scored as presence–absence (p/a) (not area).

Habitat selection by song thrushes in stable and declining farmland populations

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