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Could soil degradation contribute to farmland bird declines?Links between soil penetrability and the abundance of yellowwagtails Motacilla flava in arable fields

James J. Gilroya,*, Guy Q.A. Andersonb, Philip V. Gricec, Juliet A. Vickeryd,Iain Braya, P. Nicholas Wattse, William J. Sutherlandf

aSchool of Biological Sciences, University of East Anglia, University Plain, Norwich NR4 7TJ, UKbRSPB, The Lodge, Sandy, Bedfordshire SG19 2DL, UKcNatural England, Northminster House, Peterborough PE1 1UA, UKdBritish Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UKeVine House Farm, Deeping St. Nicholas, Spalding PE11 3DG, UKfDepartment of Zoology, University of Cambridge, Cambridge CB2 3EJ, UK

A R T I C L E I N F O

Article history:

Received 21 January 2008

Received in revised form

3 September 2008

Accepted 15 September 2008

Keywords:

Agriculture

Distribution modelling

Habitat associations

Soil quality

Farmland birds

A B S T R A C T

Major changes to the extent and quality of farmland habitats, brought by the intensifica-

tion of agricultural practice, are thought to be the main factors driving declines in a suite

of farmland bird species in Europe. Recent changes in agricultural techniques have also

contributed to widespread soil degradation, arising from increased soil exposure to erosion

forces, declining soil organic content and increasing soil compaction. Although soils have a

fundamental influence on ecosystem properties, the implications of soil degradation for

farmland biodiversity have received little attention. In this study, we measure the influence

of soil conditions on the distribution of a declining insectivorous farmland bird, the yellow

wagtail Motacilla flava, relative to other habitat features in arable fields. Soil penetrability

was found to have a significant influence on the abundance of territorial yellow wagtails

at the field scale, together with crop type. Other measured habitat features had little effect

on territory abundance, including soil organic content, crop height (within preferred crop

types), field boundary habitats and availability of bare ground. Monitoring of invertebrate

abundance across 20 cereal fields revealed a significant influence of both soil penetrability

and soil organic content on aerial invertebrate capture rates. This relationship was stron-

gest during the latter part of the breeding season, implying that settling yellow wagtails

could use soil penetrability as a predictive indicator of prey abundance during the chick-

rearing period. The strong relationship between yellow wagtails and soil penetrability sug-

gests a potential causative link between soil degradation and population decline. The role

of soils in determining abundance patterns and population declines of other farmland spe-

cies may have been overlooked in previous studies.

� 2008 Elsevier Ltd. All rights reserved.

0006-3207/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.biocon.2008.09.019

* Corresponding author: Tel.: +44 7973 413669.E-mail address: james.gilroy1@googlemail.com (J.J. Gilroy).

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1. Introduction

As the dominant landscape type across many terrestrial bio-

mes, agricultural habitats play a pivotal role in the mainte-

nance of regional and local biodiversity (Green et al., 2005).

Intensive farm management practices have brought rapid

changes to agricultural landscapes during the last 50 years,

with profound consequences for farmland biodiversity

(Chamberlain and Fuller, 2000; Stoate et al., 2001; Robinson

and Sutherland, 2002). In Britain, various factors are known

to have negatively influenced agricultural biodiversity, includ-

ing the increased use of pesticides (Potts, 1986; Brickle et al.,

2000; Morris et al., 2005), removal of non-cropped habitats

(Perkins et al., 2002; Vickery et al., 2002), a shift from spring

to autumn sowing of cereals (Shrubb, 1990; Wilson et al.,

1997) and the loss of mixed arable and livestock farming

(Evans, 1997). These changes have been major drivers of de-

clines in European farmland bird populations, many of which

are now at crisis levels (Donald et al., 2001). An additional out-

come of modern intensive farming practice is the widespread

degradation of soils (Chambers et al., 1992; Stoate et al., 2001).

Whilst soil conditions are known to be a major determinant

of agricultural productivity (Bauer and Black, 1994; Lal,

1998), the influence of soil degradation on farmland biodiver-

sity has received relatively little attention. Although much re-

search has focused on the identification of bird habitat

associations in farm environments (Whittingham et al.,

2007), little is known about the influence of soils in determin-

ing farmland bird distributions or abundance.

Soil quality is known to directly influence many organ-

isms, including a suite of soil-specialist invertebrate groups

(Schrader and Lingnau, 1997; Cortet et al., 2002). Soil compac-

tion is a strong determinant of the abundance of earthworms

and springtails (Sochtig and Larink, 1992; Larsen et al., 2004;

Stovold et al., 2004), and may also affect soil surface inverte-

brates, including beetles (Gudleifsson, 2005) and spiders

(Gudleifsson and Bjarnadottir, 2004). Soil composition and

moisture are also known to influence pupation rates and lar-

vae survival of terrestrial (Ellis et al., 2004) and aerial insects

with soil-dependent life stages (Dimou et al., 2003; Hulthen

and Clarke, 2006). However, relationships between soil varia-

tion and other taxa are rather poorly documented. Soil pene-

trability can influence birds that specialise in feeding on soil

invertebrates, such as lapwing Vanellus vanellus (Lister, 1964),

snipe Gallinago gallinago (Green et al., 1990) and song thrush

Turdus philomelos (Peach et al., 2004). Additionally, soil produc-

tivity can be correlated with predator populations (e.g. spar-

rowhawk Accipiter nisus, Newton et al., 1986). A small

number of studies have also indicated that soil variation

could influence the distributions of insectivorous passerines

(e.g. Wilson et al., 2005). However, very little attention has

been paid to the potential for soil degradation to be a causal

factor in bird population declines. Furthermore, most bird

habitat association studies20 fail to take soil variation into ac-

count (Wilson et al., 2005).

Agricultural activities can have various negative effects on

soil quality. Mechanical and chemical erosion, occurring lar-

gely when soil is exposed to wind and rainfall, can lead to dra-

matic losses of organic matter (Stoate et al., 2001), with

consequent effects on soil productivity (de la Rosa et al.,

2000), drainage, pesticide leaching (Stoate et al., 2001) and

susceptibility to soil compaction (Soane, 1990). Many aspects

of modern farming increase soil exposure to erosion pro-

cesses, including autumn sowing, declining use of grass leys,

use of irrigation, increases in field size and continuous crop-

ping (Bowman et al., 1999; Boardman et al., 2003; Evans,

2005). Declining numbers of livestock within arable systems

also lead to reductions in soil organic content (Drinkwater

et al., 1998). Pretty et al. (2000) estimated organic matter losses

of 1.7% or more across a fifth of UK soils since 1980, amount-

ing to 1.4 tonnes ha�1 year�1. In England, the proportion of

fine-textured topsoils in the lowest organic content class

(<2.3%) rose from 40% to 50%, between 1980 and 1995 (King

et al., 2005). The use of heavy machinery exacerbates soil

compaction, which may further inhibit drainage, affect nutri-

ent cycling and reduce penetrability (Hamza and Anderson,

2005). Synergistically, these changes could have a variety of

effects on wider biological communities within farmed

environments.

Like many farmland birds in Europe, populations of the

near-endemic British subspecies of yellow wagtail Motacilla

flava flavissima have undergone dramatic reductions in recent

decades. During the breeding season, abundance declined by

65% between 1972 and 2005, leading to this species’ inclusion

on red list of birds of conservation concern in the UK, as well

as the recently-revised UK Biodiversity Action Plan priority

species list (Eaton et al., 2006). It is also one of the 19 species

in the Government’s Farmland Bird Indicator, used as a mea-

sure of success of its policies for conserving biodiversity in

the countryside (Vickery et al., 2004a). A significant proportion

of the UK population breeds in the arable-dominated low-

lands of eastern England (Gibbons et al., 1993), wintering in

sub-Saharan Africa. In this paper, we explore factors affecting

patterns of yellow wagtail abundance within intensively

managed arable farmland, relating territory abundance to a

suite of habitat variables at a field-by-field scale. In particular,

we consider the relationship between soil characteristics and

wagtail territory abundance. We also examine the influence

of habitat features on the abundance of potential invertebrate

prey groups. Our findings are discussed in relation to their

implications for the conservation of yellow wagtails and other

declining insectivorous farmland birds.

2. Methods

2.1. Study area and species

The study was carried out in six areas of exclusively arable

farmland in Lincolnshire and Cambridgeshire, covering

33 km2 across 14 different farms. Table 1 shows a summary

of the main habitat features of each of the six areas. All farms

included in the study were under intensive management, and

all arable fields received broad-spectrum herbicide, fungicide

and fertiliser treatment during the study period. Cropped land

occupied a minimum of 80% of the total area on each site.

Soils in the region vary from peat-rich soil (largely in the west)

to heavy clay-rich soil. On a local scale, these soil types merge

to create a patchy mosaic of soil conditions. Most field bound-

aries consist of drainage ditches, which maintain a regionally

uniform water table. Vegetation in the ditches is typically

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controlled by a twice annual cutting regime. Most field bound-

aries also include grass strips, while hedges are relatively

infrequent (<20% of all boundaries).

Yellow wagtails are obligate insectivores, foraging either

on the ground or above the crop sward within arable fields.

As a ground-nesting species, territory requirements include

a suitable level of vegetation to conceal a nest, together with

sufficient access to the ground for foraging (Dittberner and

Dittberner, 1984). Bradbury and Bradter (2004) found that ter-

ritories in seasonally flooded grasslands tended to be associ-

ated with heterogeneous swards, bare earth and water

features such as pools. Previous studies in arable environ-

ments have found associations with more open landscapes,

as well as preferences for spring sown crops, particularly

potatoes, peas and beans (Mason and MacDonald, 2000),

and autumn-sown cereals (Stiebel, 1997).

2.2. Territory and habitat mapping

Sites were surveyed twice per month during the 2005 breeding

season (April–August), using territory mapping methodology

(Marchant, 1983). Census routes followed field boundaries or

tramlines within crops, such that all areas of a field were vis-

ited to within 50 m. Males perform advertisement songs and

song-flight displays within a small territory prior to nest

establishment, and both sexes perform aggressive mobbing

behaviour around the active nest (Smith, 1950). Consequently,

both of these behaviours were used to identify the presence of

a territory. Previous work has demonstrated that some crops

are strongly avoided by breeding yellow wagtails, including

winter-sown oilseed rape, linseed, sugar beet and set aside

(Stiebel, 1997; Gilroy, 2006). Owing to their low likelihood of

supporting territories, these crops were excluded from both

bird and habitat surveys. Crop types covering less than

100 ha in total within the study area were also excluded.

The remaining sample consisted of 101 fields of four crop

types: autumn-sown wheat, potatoes, field peas and field

beans.

Habitat features were mapped in 2005 on a field-by-field

basis. Crop height was measured (to nearest 5 cm) on each

visit at five locations per field using a 2 m measuring stick.

Field boundary types were classified by estimation of the pro-

portion of each boundary consisting of hedges, wet ditches

and bare ground (including tracks, paths and roads). Field size

and boundary length were measured using MapInfo v6.5.

Measures of soil penetrability and soil organic content were

used to characterise soil conditions within a field. Organic

content is a major determinant of both soil structure and fer-

tility, having a strong influence on the productivity and func-

tioning of agricultural soils (Reeves, 1997). Soil surface

penetrability provides a functional indicator of key soil struc-

tural characteristics, provided that comparative measures are

made under similar soil moisture conditions (El Titi, 2003).

Penetrability is often positively correlated with organic con-

tent (Huntington et al., 1989; Reeves, 1997).

Soil organic content was measured using samples taken

from within 10 cm of the soil surface from 25 random loca-

tions across each field, analysed by Anglian Soil Analysis

Ltd. (Boston, Lincolnshire). Soil penetrability was measured

using an 8 mm diameter soil penetrometer at 20 evenly

spaced points along a 100 m transect diagonally bisecting

the field. As soil compaction increases dramatically in tractor

tramlines and headlands (the outer 20 m field boundary sec-

tion), measurements were not taken from these areas. Soil

penetrability is known to vary in relation to soil moisture,

and hence antecedent weather conditions (Green, 1988). Con-

sequently, penetrability measures from all fields in the study

were taken on the same date, with the survey repeated on two

occasions, separated by three weeks, in June and July 2005.

Readings were taken between 09:00 and 18:00 to minimise po-

tential effects of diurnal variation in soil moisture levels.

2.3. Invertebrate sampling

In order to explore the influence of soil conditions on prey

availability for yellow wagtails, invertebrate abundance was

sampled in a subset of 20 autumn-sown cereal fields within

the study region in 2006. Logistical constraints prevented

sampling from any other crop types. All sampled fields were

treated with broad-spectrum herbicide, fungicide and fertil-

iser during the course of the study; insecticides were not used

on any sample fields during the study period. Yellow wagtail

diet is known to be highly variable, though most studies re-

port a dominance of aerial-active invertebrates during the

breeding season (Davies, 1977; Roselaar, 1988). Two sampling

methods were used: pitfall trapping for surface active ground

invertebrates (e.g. beetles Coleoptera and spiders Aranidae) and

sticky traps for aerial invertebrates (e.g. flies Diptera). All traps

were deployed within the crop itself, at distances exceeding

15 m from any field edge in order to control for boundary

Table 1 – Characteristics of study sites in the Fenland region of Lincolnshire and Cambridgeshire

Site Area (km2) Field size(ha) (mean ± SD)

Main soiltype

Area (ha) of:

Wheat Beans Peas Potatoes Sugar beet Oilseed rape

Archer’s drove 5.2 17.6 ± 6.5 Clay/loam 229 98 0 0 0 105

Borough fen 4.4 12.2 ± 5.5 Peat/clay/loam 129 36 40 47 90 36

Bourne south fen 6.1 12.6 ± 4.3 Peat 186 0 125 98 64 0

Deeping fen 4.5 22.1 ± 7.9 Clay/loam 215 80 0 0 75 30

Dunsby fen 9.2 15.2 ± 6.1 Clay/loam 372 165 120 0 0 170

Langtoft fen 3.9 13.3 ± 4.2 Peat/clay/loam 142 50 70 22 40 60

Soil types were identified using England and Wales Soil Survey soil categorization (Sheet 4, 1983) as follows: Clay = categories 8.13, 8.15;

Loam = 5.1, 5.3; Peat = 8.51, 10.24. Wheat crops on all sites were autumn-sown. Excluded crops (<100 ha) were linseed (20 ha), sunflowers

(18 ha), autumn-sown barley (40 ha) and set aside (85 ha).

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effects. In each sample field, two rows of five 7 cm diameter

pitfall traps were deployed, spaced at 2 m intervals in order

to maintain sample independence (Ausden, 1996). Six aerial

sticky traps (10 · 20 cm) were placed in each field using flexi-

ble fibreglass canes. Sticky traps were placed at 10 cm above

the height of the crop. All traps were checked on a weekly ba-

sis from May 5th to June 18th 2006, and the number of inver-

tebrates captured was recorded. Invertebrates that were

deemed unlikely to form part of the yellow wagtail diet, for

example those with body length below 2 mm or large bumble

bees Bombus spp (Davies, 1977), were excluded from subse-

quent analyses. All captured invertebrates were identified to

order. All habitat variables were measured again in 2006 for

each field in which invertebrate sampling took place.

2.4. Statistical analysis

Our modelling rationale followed the information theoretic

paradigm of Burnham and Anderson (2002). General linear

models (GLMs) were used to relate habitat features to the dis-

tribution of yellow wagtail territories. Site effects were ac-

counted for by including a fixed six-level categorical site

factor in all models. As habitat association models are

strongly influenced by the scale at which both habitat and re-

sponse variables are measured, every effort was made to

measure both sets of variables at the scale at which key hab-

itat selection decisions are made (Rushton et al., 2004). Yellow

wagtails strongly avoid nesting in non-cropped habitats with-

in the study environment, and concurrent work within the

same study area found that 98% of nests (n = 136) were placed

within crop fields at distances greater than 40 m from any

boundary (Gilroy, 2006). Consequently, habitat selection could

be most meaningfully modelled on a field-by-field basis. The

number of territories recorded per field was therefore mod-

elled as a response variable, with loge (field size) included as

an offset variable. In order to model the effect of crop height,

we used least-squares linear regression of log transformed

mean height measures from all visits to predict the mean

crop height in each field on a single date (May 20th). Assum-

ing that growth phenology was similar across all fields within

each crop type, this approach allowed meaningful between

field comparisons of crop height during the peak period of

settlement, controlling for variation in the dates of crop mea-

surement at each site.

The relative homogeneity of the study environment al-

lowed characterisation of field features using a relatively

small number of variables: mean soil penetrability, mean soil

organic content (%), crop type (categorical), proportion of the

field bounded by hedges/trees and wet ditches, crop height

and area of bare ground (including adjacent tracks/roads).

GLMs with Poisson errors and log link function were fitted

to the data using all possible combinations of these predictor

variables (giving 127 competing models). Each model was

ranked by its Akaike weight, using AIC values corrected for

small sample sizes (AICc). Selection probabilities were calcu-

lated for each variable by summing the Akaike weights of

all models containing that particular variable. Model-aver-

aged parameter coefficients were calculated using weighted

averages from the entire model set. Model averaging was also

used to generate unconditional standard errors for these coef-

ficients (Gibson et al., 2004). Overall model fit was assessed by

examination of the variance inflation factor c (calculated as

the ratio of residual deviance to residual degrees of freedom)

from a fully parameterised model, with values close to unity

indicating well fitting models (Burnham and Anderson,

2002). We used univariate binary logistic regression models

to predict field occupancy, calculating the area under the re-

ceiver operating characteristic (ROC) curve, which is analo-

gous to the proportion of cases correctly classified by the

model (Fielding and Bell, 1997). The prediction accuracy of

each individual variable was then assessed in relation to the

performance of the fully parameterised model.

The influence of soil characteristics on invertebrate abun-

dance was modelled using mean weekly capture rates of po-

tential prey types from pitfall traps and sticky traps. The

mean number of individuals caught per week was calculated

for each sampling method in each field, and these values were

modelled in GLMs with Poisson error distribution and log link

function, in relation to all possible combinations of our field

scale habitat variables from 20 sampled fields (measured in

2006). Again, all models included a categorical fixed effect

for site. From these models, Akaike weights and variable

selection probabilities were calculated in order to determine

the magnitude and plausibility of each effect.

3. Results

3.1. Yellow wagtail distribution

Yellow wagtail territories were found in all four crop types

surveyed (Table 2). Within these crops, we found a strong uni-

variate relationship between yellow wagtail distribution and

soil penetrability. Fields with more penetrable soils tended

to have a higher density of territories (Fig. 1). The significant

role of soil penetrability in determining wagtail abundance

was supported by multivariate statistical analysis. The 15 best

Table 2 – Habitat characteristics and territory densities across four principal crop types used by territorial yellow wagtailsin 2005

Crop Soil penetrability(g cm�2)

Soil organic content(%)

Crop height on 20th May(cm)

Territory density(males km�2)

Autumn-sown wheat 32.71 ± 14.66 18.36 ± 11.25 63.9 ± 6.27 1.43 ± 1.41

Field beans 21.70 ± 6.01 13.89 ± 8.61 72.6 ± 17.75 3.21 ± 2.20

Peas 17.35 ± 3.75 23.40 ± 9.51 29.1 ± 15.13 3.33 ± 0.71

Potatoes 21.71 ± 7.21 27.57 ± 9.65 15.7 ± 6.57 4.86 ± 3.28

Values are means across all fields in the study area (±SD).

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ranked models in explaining variation in territory abundance

from all combinations (i.e. 127 competing models) of our pre-

dictor variables are shown in Table 3. Soil penetrability was

included in all of the best fitting models, although no single

model was superior to others in explaining variation in terri-

tory abundance, indicating some uncertainty in the level of

influence of other measured variables.

Penetrability and soil organic content were positively cor-

related (Pearson R2 = 0.249, P < 0.05), but organic content per-

formed poorly in predicting territory abundance, appearing

in only six of the best 15 models. Yellow wagtails therefore

appear to respond to variation in penetrability more closely

than changes in soil organic matter. Selection probabilities

for each variable are shown in Table 4. Soil penetrability had

the highest selection probability (>0.99), suggesting very

strong support for the effect of this variable. Crop type also

had a high selection probability (0.837). The lower selection

probabilities of other variables suggest that their effect on ter-

ritory abundance was weak.

The AIC differences between the 15 best ranked models

were small (<3), and therefore model averaging was a suitable

method for the estimation of coefficients, which are shown in

Table 4, together with their unconditional standard errors.

Within the crop types considered in this survey, all of which

were known to be broadly favoured by yellow wagtails, stron-

gest preference was shown for potatoes and field beans, fol-

lowed by winter-sown wheat and field peas. There was also

a weakly supported negative association with the presence

of hedges, although the selection probability associated with

this variable was very low (0.485), suggesting considerable

uncertainty in this relationship. The power of each individual

variable to predict field occupancy is shown in Table 5. The

area under the ROC curve is analogous to the proportion of

fields correctly classified (as occupied or unoccupied) by uni-

variate models including each individual variable. Prediction

accuracy was highest for the soil penetrability model, which

correctly classified 86.3% of cases. Indeed, the prediction

accuracy of this univariate model was only fractionally lower

Fig. 1 – Relationship between soil penetrability and yellow wagtail territory density in fields across six study areas in eastern

England in 2005. Error bars indicate standard deviation across 20 soil penetrability sampling points in each field. ‘‘Other

crops’’ were potatoes, field peas and field beans. Crops were excluded from the survey if they were known to have a low

likelihood of holding yellow wagtail territories, including winter-sown oilseed rape, sugar beet and set aside fields. Soil

penetrability was the highest-ranked explanatory variable in multivariate models predicting territory abundance (AIC

selection probability >0.99).

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than that of the fully parameterised model (88.3%). The

remaining variables all had poor prediction accuracies (<65%).

3.2. Invertebrate abundance

Weekly invertebrate capture rates from both sticky traps and

pitfall traps increased during the course of the sample period,

with a concurrent increase in the magnitude of variation be-

tween fields (Fig. 2). Mean weekly capture rates from each

field were modelled in relation to all combinations of habitat

variables, with values for early (May) and late (June) periods

modelled separately. In all comparisons, variation in model

explanatory power (measured by AICc) was small, indicating

considerable uncertainty in determining the best model to

predict variation in invertebrate abundance. Capture rates of

aerial insects from sticky traps were found to be positively

influenced by soil penetrability in June, but not in May (Table

6, Fig. 3). Soil organic content had a positive influence on the

abundance of aerial insects in both months, although the

relationship was stronger in June (variable selection probabil-

ities 0.98 (May) and >0.99 (June), see Table 6). Soil organic con-

tent explained 35% of variation in mean aerial insect capture

rates from sticky traps in June (Fig. 3); fields with high organic

content tended to have the highest late-season peaks in aerial

insect abundance (Fig. 2). Model selection for May also sug-

gested a positive relationship between aerial insect abun-

dance and the proportion of a field bounded by wet ditches

(selection probability 0.90, Table 6), and a weak positive rela-

tionship with hedges (selection probability 0.76, Table 6),

although these relationships were less apparent in June.

Table 3 – Results of model selection for the candidate model set predicting yellow wagtail territory abundance, showingthe 15 best ranking models from all candidate models considered (n = 127), their AIC differences (DAIC) and modelselection probabilities (AICw)

Rank Variables AICc Di AICw

1 Soilpen + crop + hedge 297.1 0.000 0.090

2 Soilpen + crop 297.2 0.083 0.087

3 Soilpen + organic% + crop + hedge 298.3 1.143 0.051

4 Soilpen + organic% + crop 298.5 1.356 0.046

5 Soilpen + crop + hedge + height 298.6 1.462 0.044

6 Soilpen + crop + height 298.6 1.518 0.042

7 Soilpen + crop + bare 298.7 1.598 0.041

8 Soilpen + crop + bare + hedge 298.8 1.724 0.038

9 Soilpen + wetditch + crop + hedge 299.1 1.942 0.034

10 Soilpen + height 299.1 1.989 0.033

11 Soilpen + wetditch + crop 299.1 2.004 0.033

12 Soilpen + organic% + crop + hedge + height 299.4 2.232 0.030

13 Soilpen + organic% + crop + height 299.6 2.441 0.027

14 Soilpen + organic% + crop + bare 299.9 2.781 0.023

15 Soilpen + organic% + crop + bare + hedge 299.9 2.796 0.022

Note that all models included ‘‘site’’ as a fixed effect, as well as loge(field size) as an offset variable. Variable descriptions: soilpen = force

required to penetrate soil; organic% = soil organic content; crop = crop type (categorical); hedge = proportion of field bordered by hedge;

bare = area of bare ground (including tracks/roads); wetditch = proportion of field bordered by water-filled ditches; height = crop height on 20th

May.

Table 4 – Selection probabilities, model-averaged coeffi-cients and unconditional standard errors for all variablesconsidered in models predicting yellow wagtail territoryabundance

Variable Selectionprobability

Coefficient Standarderror

Force required to penetrate

soil

>0.999 �0.089 0.011

Crop type (peas) 0.837 �0.191 0.190

Crop type (potatoes) 0.837 0.905 0.310

Crop type (winter wheat) 0.837 �0.054 0.089

Crop type (beans) 0.837 0.000

Hedge 0.485 �0.156 0.128

Crop height 0.445 0.004 0.004

Soil organic content 0.366 0.023 0.031

Bare ground 0.313 0.051 0.058

Wet ditches 0.274 0.003 0.081

Selection probabilities are summed AICw values for all models

including a given variable, and represent a probabilistic indicator of

the plausibility of the effect. Coefficients for levels of the categor-

ical variable ‘‘Crop’’ are measured relative to ‘‘beans’’.

Table 5 – Accuracy of univariate binary logistic models inpredicting field occupancy by yellow wagtails, togetherwith the full model including all variables

Variable Area under ROC curve

Soil penetrability 0.863

Crop type 0.637

Hedge 0.551

Crop height 0.587

Organic% 0.528

Bare ground 0.518

Wet ditches 0.570

Full model 0.883

The area under the receiver operating characteristic (ROC) curve is

analogous to the proportion of fields correctly classified by the

model.

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None of the habitat variables considered here had a strong

influence on invertebrate capture rates in pitfall traps (Table

6). There was tentative support for a positive influence of in-

creased soil penetrability on the abundance of captures

excluding beetles (mainly spiders) in both May and June,

whilst beetle abundance was also weakly associated with

the proportion of fields bordered by both hedges and ditches

in June (Table 6). However, the low selection probabilities of

all variables considered in these models imply that variation

in abundance of these terrestrial invertebrate groups is prin-

cipally determined by factors that were not explicitly consid-

ered in this study.

4. Discussion

Within the four crop types used by nesting yellow wagtails in

our study area, soil penetrability was the strongest determi-

nant of territory abundance. Indeed, this single variable had

strong explanatory power in predicting field occupancy. Soil

organic content, by comparison, was a poor predictor of terri-

tory abundance, despite being directly related to soil penetra-

bility (Huntington et al., 1989; Da Silva et al., 1997). The close

association between soil penetrability and yellow wagtail

abundance is surprising, given the lack of any obvious mech-

anism linking soil structure and wagtail breeding ecology. A

number of previous studies have found relationships between

bird distributions and variation in soil penetrability, but these

cases have invariably involved species that feed by probing

into soil, such as snipe (Green et al., 1990), or those that prey

heavily on soil organisms, such as song thrushes feeding on

earthworms (Peach et al., 2004). Yellow wagtails forage by aer-

ial flycatching, picking or gleaning (Davies, 1977; Gilroy et al.

unpublished data), and soil conditions are unlikely to affect

their foraging efficiency. Soil penetrability may, however, be

an important determinant of prey abundance for this species

within arable fields.

We found both soil penetrability and soil organic content

to be significant predictors of variation in the abundance of

aerial insects within cereal crops, sampled using sticky traps,

although neither variable was found to influence capture

rates of terrestrial invertebrates in pitfall traps. Many of the

invertebrate groups sampled by sticky traps are likely to fea-

ture in the yellow wagtail diet, including flies, Hymenoptera,

aerial-active beetles and Lepidoptera (Roselaar, 1988). Interest-

ingly, the relationship between soil characteristics and aerial

invertebrate abundance became much stronger during the

chick-rearing period (June), when invertebrate abundance

peaked. It is therefore possible that, during settlement, yellow

wagtails assess soil conditions as a predictive indicator of

Fig. 2 – Mean weekly capture rates of aerial insects from

sticky traps in intensively managed autumn-sown wheat

fields (samples from six traps per field across 20 fields)

during May and June 2006. Fields are categorised according

to the mean soil organic content from 25 samples taken

across the surface layer of each field. Bars indicate standard

errors. Soil organic content was the highest-ranked

explanatory variable in multivariate models predicting

aerial insect abundance (AIC selection probability 0.98).

Table 6 – Selection probabilities, model-averaged coefficients and unconditional standard errors for all variablesconsidered in models of invertebrate capture rates from sticky traps and pitfall traps in autumn-sown wheat fields in Mayand June

Response variable Predictor variable May June

AIC selectionprobability

Coefficient ± SE AIC selectionprobability

Coefficient ± SE

Aerial invertebrates

(sticky traps)

Force required to penetrate soil 0.34 – >0.99 �0.009 ± 0.002

Soil organic content 0.98 0.005 ± 0.002 >0.99 0.007 ± 0.001

Hedge boundaries 0.76 0.270 ± 0.165 0.39 –

Wet ditch boundaries 0.90 0.688 ± 0.115 0.46 –

Crop height 0.31 – 0.34 –

Coleoptera (pitfall

traps)

Force required to penetrate soil 0.65 �0.009 ± 0.008 0.31 –

Soil organic content 0.30 – 0.39 –

Hedge boundaries 0.40 – 0.64 0.139 ± 0.218

Ditch boundaries 0.52 0.202 ± 0.328 0.72 0.461 ± 0.221

Crop height 0.30 – 0.30 –

Other terrestrial

Invertebrates

(pitfall traps)

Force required to penetrate soil 0.56 �0.007 ± 0.020 0.78 �0.030 ± 0.016

Soil organic content 0.32 – 0.41 –

Hedge boundaries 0.33 – 0.44 –

Ditch boundaries 0.32 – 0.44 –

Crop height 0.30 – 0.33 –

Coefficients are given only for variables with selection probabilities exceeding 0.5.

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prey availability during the key chick-rearing period, a month

or so after arrival. Such strategies arise frequently in cases

where organisms are required to make habitat selection deci-

sions at times when future habitat quality is not directly evi-

dent (Stamps and Krishnan, 2005).

Our models suggest that soil organic content had a stron-

ger influence on aerial insect abundance than soil penetrabil-

ity, being significantly related to sticky trap capture rates in

both May and June. It is therefore surprising that yellow wag-

tail abundance was more closely related to variation in soil

penetrability, despite the two characteristics being closely

correlated. It is possible that soil penetrability (or a correlated

structural feature) is more easily appraised by wagtails than

organic content itself, and is therefore used as a settlement

cue despite being a potentially poorer predictor of food avail-

ability. Another possibility is that yellow wagtails favour a

narrow range of specific invertebrate prey groups, and these

groups are more strongly influenced by soil structure (and

hence penetrability) than soil organic content. We were not

able to analyse diet preferences in this study, and yellow wag-

tail diet composition is known to vary widely between study

areas (see Davies, 1977; Roselaar, 1988). A further possibility

is that both penetrability and organic content are correlated

with other soil features that influence prey abundance, for

example soil moisture content. Soil structure and composi-

tion have a strong influence on moisture retention, which

can in turn influence soil fauna (Dimou et al., 2003; Hulthen

and Clarke, 2006). Seasonal soil moisture levels could there-

fore play an additional role in determining habitat quality

for yellow wagtails, although the measurement of this vari-

able was beyond the scope of this study.

Higher soil penetrability could confer other non-foraging

benefits to nesting yellow wagtails. Several ground-nesting

species are known to be strongly influenced by the suitability

of sites for nest construction, e.g. stone curlew Burhinus oedi-

cnemus (Green and Griffiths, 1994) and skylark Alauda arvensis

(Odderskær et al., 1997). As yellow wagtails excavate a small

scrape for nesting (Smith, 1950), more penetrable soils could

facilitate easier or better construction of this nest scrape. In

addition, within fields of a similar crop type, soils can have

a major influence on microclimatic conditions (Hunkar,

2002): nests on more penetrable (and hence more aerated)

soils might enjoy warmer temperatures and better drainage,

potentially improving the likelihood of survival to fledging.

A final possibility is that soil penetrability is correlated with

an unknown and unmeasured factor. We used soil penetrabil-

ity as an indicator of soil structure, as it is known to be di-

rectly related to other soil features including plant and

mycorrhizal growth rates (Nandian et al., 1997), water reten-

tion (Huntington et al., 1989) and response to agrochemical

treatments (Stoate et al., 2001), all of which may be important

in themselves. Further studies are needed in order to shed

light on these possibilities, as well as determine the generality

of the influence of soils on other insectivorous farmland

birds.

Our study revealed a number of other habitat associations

shown by yellow wagtails, including a marked preference for

potato and field bean crops, with field peas and autumn-sown

wheat being preferred to a lesser extent, as inferred from

model parameters. This pattern broadly concurs with other

published crop associations for the species (Mason and Mac-

donald, 2000; Stiebel, 1997). Yellow wagtails are known to be

associated with open landscapes (Stiebel, 1997), as implied

by the weak negative relationship with hedgerows in this

study. The lack of a strong relationship with wet ditches is

perhaps surprising, given that wet habitats are known to pro-

vide key resources for many other farmland species (Bradbury

and Kirby, 2006). Furthermore, previous studies in grasslands

have found close foraging associations between yellow wag-

tails and water features (Davies, 1977; Bradbury and Bradter,

2004). In our study area, wet landscape features may influ-

ence yellow wagtail distribution on a larger spatial scale than

considered here, as adult insects originating from wet habi-

tats may be able to travel large distances and hence still be

available as prey items to breeding wagtails.

4.1. Conservation implications

Soil conditions appear to exert a strong influence on yellow

wagtail abundance and distribution within our study environ-

ment, and may be a major determinant of prey abundance. It

is therefore possible that local and regional variation in soils

have an influence on yellow wagtail abundance at a popula-

tion level. Yellow wagtails occur across a variety of soil types

and agricultural habitats in eastern Britain (Gibbons et al.,

1993), and the influence of soil variation across the remainder

of this range is unknown. However, given the strength of the

Fig. 3 – The relationship between log10mean weekly capture

rates of aerial invertebrates per sticky trap and soil

characteristics in autumn-sown wheat fields during June

2006. Points represent mean weekly capture rates from

individual fields (from six traps per field) in relation to mean

soil penetrability (upper, AIC selection probability 0.90) and

mean soil organic content (lower, AIC selection probability

0.98) from each field.

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relationship found within our study area, a causal relation-

ship could exist between on-going agricultural soil degrada-

tion and population declines in this species. Moreover, very

few previous studies have considered soil variation when

modelling bird habitat associations, and the role played by

soils in influencing other declining insectivorous farmland

species is largely unknown.

Diminishing organic content and increasing soil compac-

tion are two of the main consequences of soil degradation

resulting from agricultural practice (Stoate et al., 2001). In

our study region, agricultural change has followed a pattern

typical of many arable-dominated areas (Chamberlain and

Fuller, 2000; Stoate et al., 2001), with some parts of the agricul-

tural plain having experienced vertical losses of up to 2.5 m

during the last 200 years (Coles and Hall, 1996), mainly from

erosion of the organic-rich surface layer. The widespread cul-

tivation of crops associated with intensive usage of machin-

ery and irrigation, including legumes, vegetable crops and

potatoes, is likely to have contributed significantly to degra-

dation rates (Horn et al., 1995). Centres of yellow wagtail

abundance in the region are currently associated with areas

of organic-rich soil (Gilroy, 2006), suggesting that this species

may have been more abundant in the region prior to this

widespread soil erosion. The future of remaining populations

in the region may be dependent, in part, on the protection of

existing soils from further degradation.

Resource protection and soil conservation are now major

themes in both EU and UK environmental policy (<http://

www.defra.gov.uk/wildlife-countryside/natres/index.htm>),

although the main emphasis is to preserve the economic

functioning of the soil. For most arable farming purposes,

the economic viability of cultivation is unlikely to be im-

pacted unless organic content declines to below 2% (Loveland

and Webb, 2003). Most arable topsoils in the UK remain above

this level (Anonymous, 2004), including all those encountered

in this study. The apparent sensitivity to soil penetrability

shown by yellow wagtails suggests that soil degradation

might impact this species before major economic effects are

felt in agriculture. As a species of high conservation priority

in the UK (Eaton et al., 2006), consideration should be given

to the potential for soil degradation to contribute to future

population declines. More importantly, information on soil

associations should play a key role in the regional and local

targeting of conservation actions for the recovery of this

species.

Various land management practices are known to have

beneficial effects on agricultural soils, including minimum

tillage (Lal and Kimble, 1997), organic amendment and green

manure (Grandy and Robertson, 2006), and the sowing of

rye-grass catch crops (Breland, 1995). Such measures are al-

ready advocated widely in order to protect soil integrity, but

particular focus should be placed on associating remedial

activities with cropping practices that are known to exacer-

bate soil degradation, including the intensive use of heavy

machinery, autumn sowing and irrigation (Horn et al., 1995;

Boardman et al., 2003; Evans, 2005). In England, the develop-

ment of a soil management plan is an option within the Envi-

ronmental Stewardship Entry Level Scheme (option EM1) and

soil structure is also considered within the Farm Environment

Plan required as part of the Higher Level Scheme (<http://

www.defra.gov.uk/erdp/pdfs/es/es-promotional-book-

let.pdf>). If they are adopted widely and implemented appro-

priately, these measures could result in a ‘win-win’,

combining biodiversity benefits with broader advantages for

local environments and agriculture, including protection of

water resources (Vickery et al., 2004b). In this study, we

demonstrate that soil conditions can have a strong influence

on the suitability of farm habitats for a bird species of conser-

vation concern. We recommend that future studies of farm-

land species aim to consider the potential influence of soil

characteristics on target species, at both local and landscape

scales.

Acknowledgements

We thank Richard Bradbury, David Buckingham, Dan Cham-

berlain, Chris Stoate and three anonymous referees for help-

ful comments on this manuscript. We also thank Ute Bradter,

Michal Maniakowski and David Gilroy for fieldwork, as well as

the land owners who allowed access to their farms. This work

was funded by Natural England, the RSPB, the British Trust for

Ornithology and Anglia Water.

R E F E R E N C E S

Anonymous, 2004. The state of the UK’s Birds 2004. British Trustfor Ornithology, UK.

Ausden, M., 1996. Invertebrates. In: Sutherland, W.J. (Ed.),Ecological Census Techniques. Cambridge University Press,Cambridge, UK, pp. 139–177.

Bauer, A., Black, A.L., 1994. Quantification of the effect of soilorganic matter content on soil productivity. Soil ScienceSociety of America Journal 58, 185–193.

Boardman, J., Poesen, J., Evans, R., 2003. Socio-economic factors insoil erosion and conservation. Environmental Science andPolicy 6, 1–6.

Bowman, R.A., Vigil, M.F., Nielsen, D.C., Anderson, R.L., 1999. Soilorganic matter changes in intensively cropped drylandsystems. Soil Science Society of America Journal 63, 186–191.

Bradbury, R.B., Bradter, U., 2004. Habitat associations of yellowwagtails Motacilla flava flavissima on lowland wet grassland.Ibis 146, 241–246.

Bradbury, R.B., Kirby, W.B., 2006. Farmland birds and resourceprotection in the UK: cross-cutting solutions for multi-functional farming? Biological Conservation 129, 530–542.

Breland, T.A., 1995. Green manuring with clover and ryegrasscatch crops undersown in spring wheat: effects on soilstructure. Soil Use and Management 11, 163–167.

Brickle, N.W., Harper, D.G.C., Aebischer, N.J., Cockayne, S.H., 2000.Effects of agricultural intensification on the breeding successof corn buntings Miliaria calandra. Journal of Applied Ecology37, 742–755.

Burnham, K.P., Anderson, D.R., 2002. Model Selection andMultimodel Inference: A Practical Information–TheoreticApproach, second ed. Springer, New York.

Chamberlain, D.E., Fuller, R.J., 2000. Local extinctions and changesin species richness of lowland farmland birds in England andWales in relation to recent changes in agricultural land-use.Agriculture, Ecosystems and Environment 78, 1–17.

Chambers, B.J., Davies, D.B., Holmes, S., 1992. Monitoring of watererosion on arable farms in England and Wales, 1989–90. SoilUse and Management 8, 163–170.

3124 B I O L O G I C A L C O N S E R V A T I O N 1 4 1 ( 2 0 0 8 ) 3 1 1 6 – 3 1 2 6

Author's personal copy

Coles, J., Hall, D., 1996. Changing Landscapes: The AncientFenland. Cambridgeshire County Council, Cambridge.

Cortet, J., Ronce, D., Poinsot-Balaguer, N., Beaufreton, C., Chabert,A., Viaux, P., Cancela de Fonseca, J.P., 2002. Impacts of differentagricultural practices on the biodiversity of microarthropodcommunities in arable crop systems. European Journal of SoilBiology 38, 239–244.

Da Silva, A.P., Kay, B.D., Perfect, E., 1997. Management versusinherent soil properties effects on bulk density and relativecompaction. Soil and Tillage Research 44, 81–93.

Davies, N.B., 1977. Prey selection and social behaviour in wagtails(Aves-Motacillidae). Journal of Animal Ecology 46, 37–57.

de la Rosa, D., Moreno, J., Mayol, F., Bonson, T., 2000. Assessmentof soil erosion vulnerability in western Europe and potentialimpact on crop productivity due to loss of soil depth using theImpelERO model. Agriculture Ecosystems and Environment81, 179–190.

Dimou, I., Koutsikopoulos, C., Economopoulos, A.P., Lykakis, J.,2003. Depth of pupation of the wild olive fruit fly, Bactrocera(Dacus) oleae (Gmel.) (Dipt., Tephritidae), as affected by soilabiotic factors. Journal of Applied Entomology 127, 12–17.

Dittberner, H., Dittberner, W., 1984. Die Schafstelze. Neue-BrehmBuecherei, Lutherstadt Wittenberg.

Donald, P.F., Green, R.E., Heath, M.F., 2001. Agriculturalintensification and the collapse of Europe’s farmland birdpopulations. Proceedings of the Royal Society – BiologicalSciences Series B 268, 25–29.

Drinkwater, L.E., Wagoner, P., Sarrantonio, M., 1998. Legume-based cropping systems have reduced carbon and nitrogenlosses. Nature 396, 262–265.

Eaton, M.A., Austin, G.E., Banks, A.N., Conway, G., Douse, A., Grice,P.V., Hearn, R., Hilton, G.M., Hoccum, D., Musgrove, A.J., Noble,D.G., Ratcliffe, N., Rehfisch, M.M., Worden, J., Wotton, S. 2006.The State of the UK’s Birds 2006. RSPB, BTO, WWT, CCW, EHS,NE and SNH, Sandy, Beds.

El Titi, A., 2003. Soil Tillage in Agroecosytems. CRC Press, London,UK.

Ellis Jr., J.D., Hepburn, R., Luckman, B., Elzen, P.J., 2004. Effects ofsoil type, moisture, and density on pupation success of Aethinatumida (Coleoptera: Nitidulidae). Environmental Entomology33, 794–798.

Evans, A.D., 1997. The importance of mixed farming for seed-eating birds in the UK. In: Pain, D.J., Pienkowski, M.W. (Eds.),Farming and Birds in Europe: The Common Agricultural Policyand Its Implications for Bird Conservation. Academic Press,London.

Evans, R., 2005. Reducing soil erosion and the loss of soil fertilityfor environmentally-sustainable agricultural cropping andlivestock production systems. Annals of Applied Biology 146,137–146.

Fielding, A.H., Bell, J.F., 1997. A review of methods for theassessment of prediction errors in conservation presence/absence models. Environmental Conservation 24, 38–49.

Gibbons, D.W., Reid, J.B., Chapman, R.A., 1993. The New Atlas ofBreeding Birds in Britain and Ireland: 1988–1991. T. and A.D.Poyser, London, UK.

Gibson, L.A., Wilson, B.A., Cahill, D.M., Hill, J., 2004. Spatialprediction of rufous bristlebird habitat in a coastal heathland:a GIS-based approach. Journal of Applied Ecology 41, 213–223.

Gilroy, J.J., 2006. Breeding ecology and conservation of yellowwagtails (Motacilla flava) in intensive arable farmland. Ph.D.Thesis, University of East Anglia.

Grandy, A.S., Robertson, G.P., 2006. Aggregation and organicmatter protection following tillage of a previouslyuncultivated soil. Soil Science Society of America Journal 70,1398–1406.

Green, R.E., 1988. Effects of environmental factors on the timingand success of breeding of common snipe Gallinago gallinago(Aves, Scolopacidae). Journal of Applied Ecology 25, 79–93.

Green, R.E., Griffiths, G.H., 1994. Use of preferred nesting habitatof stone curlews Burhinus oedicnemus in relation to vegetationstructure. Journal of Zoology 233, 457–471.

Green, R.E., Hirons, G.J.M., Cresswell, B.H., 1990. Foraging habitatsof female Common Snipe Gallinago gallinago during theincubation period. Journal of Applied Ecology 27, 325–335.

Green, R.E., Cornell, S.J., Scharlemann, J.P.W., Balmford, A., 2005.Farming and the fate of wild nature. Science 307, 550–555.

Gudleifsson, B.E., 2005. Beetle species (Coleoptera) in hayfieldsand pastures in northern Iceland. Agriculture, Ecosystems andEnvironment 109, 181–186.

Gudleifsson, B.E., Bjarnadottir, B., 2004. Spider (Araneae)populations in hayfields and pastures in northern Iceland.Journal of Applied Entomology 128, 284–291.

Hamza, M.A., Anderson, W.K., 2005. Soil compaction in croppingsystems – a review of the nature, causes and possiblesolutions. Soil and Tillage Research 82, 121–145.

Horn, R., Domzal, H., Slowinska-Jurkiewicz, A., Van Ouwerkerk,C., 1995. Soil compaction processes and their effects on thestructure of arable soils and the environment. Soil and TillageResearch 35, 23–36.

Hulthen, A.D., Clarke, A.R., 2006. The influence of soil type andmoisture on pupal survival of Bactrocera tryoni (Froggatt)(Diptera: Tephritidae). Australian Journal of Entomology 45,16–19.

Hunkar, M., 2002. Moisture supply and microclimate-interactionswith productivity potential. Physics and Chemistry of theEarth 27, 1113–1117.

Huntington, T.G., Johnson, C.E., Johnson, A.H., Siccana, T.G., Ryan,D.F., 1989. Carbon, organic matter, and bulk densityrelationships in a forested spodosol. Soil Science 148, 380–386.

King, J.A., Bradley, R.I., Harrison, R., 2005. Current trends of soilorganic carbon in English arable soils. Soil Use andManagement 21, 189–195.

Lal, R., 1998. Soil erosion impact on agronomic productivity andenvironment quality. Critical Reviews in Plant Sciences 17,319–464.

Lal, R., Kimble, J.M., 1997. Conservation tillage for carbonsequestration. Nutrient Cycling in Agroecosystems 49, 243–253.

Larsen, T., Schjonning, P., Axelsen, J., 2004. The impact of soilcompaction on euedaphic Collembola. Applied Soil Ecology 26,273–281.

Lister, M.D., 1964. The Lapwing habitat enquiry, 1960–61. BirdStudy 11, 128–147.

Loveland, P., Webb, J., 2003. Is there a critical level of organicmatter in the agricultural soils of temperate regions: a review.Soil and Tillage Research 70, 1–18.

Marchant, J., 1983. Common Bird Census Instructions. BritishTrust for Ornithology, Tring.

Mason, C.F., Macdonald, S.M., 2000. Influence of landscape andland-use on the distribution of breeding birds in farmland ineastern England. Journal of Zoology 251, 339–348.

Morris, A.J., Wilson, J.D., Whittingham, M.J., Bradbury, R.B., 2005.Indirect effects of pesticides on breeding yellowhammer(Emberiza citrinella). Agriculture Ecosystems and Environment106, 1–16.

Nandian, H., Smith, S.E., Alston, A.M., Murray, R.S., 1997. Effects ofsoil compaction on plant growth, phosphorus uptake andmorphological characteristics of vesicular–arbuscularmycorrhizal colonization of Trifolium subterraneum. NewPhytologist 135, 303–311.

Newton, I., Wyllie, I., Mearns, R., 1986. Spacing of sparrowhawksin relation to food supply. Journal of Animal Ecology 55, 361–370.

B I O L O G I C A L C O N S E R V A T I O N 1 4 1 ( 2 0 0 8 ) 3 1 1 6 – 3 1 2 6 3125

Author's personal copy

Odderskær, P., Prang, A., Poulsen, J.G., Andersen, P.N., Elmegaard,N., 1997. Skylark (Alauda arvensis) utilisation of micro-habitatsin spring barley fields. Agriculture, Ecosystems andEnvironment 62, 21–29.

Peach, W.J., Robinson, R.A., Murray, K.A., 2004. Demographic andenvironmental causes of the decline of rural song thrushesTurdus philomelos in lowland Britain. Ibis 146 (Suppl. 2), 50–59.

Perkins, A.J., Whittingham, M.J., Morris, A.J., Bradbury, R.B., 2002.Use of field margins by foraging yellowhammers Emberizacitrinella. Agriculture, Ecosystems and Environment 93, 413–420.

Potts, G.R., 1986. The partridge: pesticides. Predation andConservation. Collins, London.

Pretty, J.N., Brett, C., Gee, D., Hine, R.E., Mason, C.F., Morison, J.I.L.,Raven, H., Rayment, M.D., Van der Bijl, G., 2000. An assessmentof the total external costs of UK agriculture. AgriculturalSystems 65, 113–136.

Reeves, D.W., 1997. The role of soil organic matter in maintainingsoil quality in continuous cropping systems. Soil and TillageResearch 43, 131–167.

Robinson, R.A., Sutherland, W.J., 2002. Post-war changes in arablefarming and biodiversity in Great Britain. Journal of AppliedEcology 39, 157–176.

Roselaar, C.S., 1988. Yellow wagtail Motacilla flava. In: Cramp, S.(Ed.), . In: Handbook of the Birds of Europe, the Middle East andNorth Africa, vol. V. Oxford University Press, London, pp. 413–433.

Rushton, S.P., Ormerod, S.J., Kerby, G., 2004. New paradigms formodelling species distributions? Journal of Applied Ecology 41,193–200.

Schrader, S., Lingnau, M., 1997. Influence of soil tillage and soilcompaction on microarthropods in agricultural land.Pedobiologia 41, 202–209.

Shrubb, M., 1990. Effects of agricultural change on nestingLapwing Vanellus vanellus in England and Wales. Bird Study 37,115–127.

Smith, S., 1950. The Yellow Wagtail. Collins, London.Soane, B.D., 1990. The role of organic matter in soil compactibility:

a review of some practical aspects. Soil and Tillage Research16, 179–201.

Sochtig, W., Larink, O., 1992. Effect of soil compaction on activityand biomass of endogeic lumbricids in arable soils. SoilBiology and Biochemistry 24, 1595–1599.

Stamps, J., Krishnan, V.V., 2005. Nonintuitive cue use in habitatselection. Ecology 86, 2860–2867.

Stiebel, H., 1997. Habitat selection, habitat use and breedingsuccess in the yellow wagtail Motacilla flava in an arablelandscape. Vogelwelt 118, 257–268.

Stoate, C., Boatman, N.D., Borralho, R.J., Carvalho, C.R., de Snoo,G.R., Eden, P., 2001. Ecological impacts of arable intensificationin Europe. Journal of Environmental Management 63, 337–365.

Stovold, R.J., Whalley, W.R., Harris, P.J., White, R.P., 2004. Spatialvariation in soil compaction, and the burrowing activity of theearthworm Aporrectodea caliginosa. Biology and Fertility of Soils39, 360–365.

Vickery, J., Carter, N., Fuller, R.J., 2002. The potential value ofmanaged cereal field margins as foraging habitats forfarmland birds in the UK. Agriculture Ecosystems andEnvironment 89, 41–52.

Vickery, J.A., Evans, A.D., Grice, P., Brand-Hardy, R., Aebischer,N.A., 2004a. Ecology and conservation of lowland farmlandbirds II: The road to recovery. Ibis 146 (Suppl. 2),1–258.

Vickery, J.A., Bradbury, R.B., Henderson, I.G., Eaton, M.A., Grice,P.V., 2004b. The role of agri-environment schemes and farmmanagement practices in reversing the decline of farmlandbirds in England. Biological Conservation 119, 19–39.

Whittingham, M.J., Krebs, J.K., Swetnam, R.D., Vickery, J.A.,Wilson, J.W., Freckleton, R.P., 2007. Should conservationstrategies consider spatial generality? Farmland birds showregional not national patterns of habitat association. EcologyLetters 10, 25–35.

Wilson, A.M., Fuller, R.J., Day, C., Smith, G., 2005. NightingalesLuscinia megarhynchos in scrub habitats in the southern fens ofEast Anglia, England: associations with soil type andvegetation structure. Ibis 147, 498–511.

Wilson, J.D., Evans, J., Browne, S.J., King, J.R., 1997. Territorydistribution and breeding success of skylarks Alauda arvensison organic and intensive farmland in southern England.Journal of Applied Ecology 34, 1462–1478.

3126 B I O L O G I C A L C O N S E R V A T I O N 1 4 1 ( 2 0 0 8 ) 3 1 1 6 – 3 1 2 6