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Population trends of Jays, Magpies, Jackdaws andCarrion Crows in the United KingdomR.D. Gregory & J.H. MarchantPublished online: 29 Mar 2010.

To cite this article: R.D. Gregory & J.H. Marchant (1996) Population trends of Jays, Magpies, Jackdaws and Carrion Crowsin the United Kingdom, Bird Study, 43:1, 28-37, DOI: 10.1080/00063659609460993

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The population levels of corvids are of spe-cial interest to birdwatchers and

conservationists alike. Several members of thisfamily have shown long-term changes in popu-lation status.1,2 Magpie Pica pica, and to a lesserextent Carrion Crow Corvus corone, can beimportant predators of eggs and nestlings ofopen-nesting songbirds and gamebirds.3�5 Ithas been suggested that their populationincreases may have been a cause of coincidentdeclines of open-nesting species, althoughthere is little direct support for this.3�5 Five ofthe 7 native British corvids, Jay Garrulus glan-darius, Magpie, Jackdaw Corvus monedula, RookCorvus frugilegus and Carrion Crow, are wide-spread, opportunistic species which come intoconflict with human interest, most notably withfarming and shooting interests, and there is aneed to monitor their status as �pest species�.

The Wildlife and Countryside Act (1981) list-ed 13 species, including the 5 corvids above,under Schedule 2, Part II. Under this legisla-tion, these species could be controlled at anyseason of the year. Recent amendments to thelegislation, however, have removed the speciesfrom this schedule to bring Britain into line

with the requirements of the EuropeanCommunity (EC). The British Government hassecured a derogation under the EC Wild BirdsDirective (EC/79/409) to allow the generallicensing of pest control over relatively largeadministrative regions. This system has been inoperation since January 1993. Licences areissued in the interests of public health and safe-ty, to prevent serious damage to crops,livestock, forests, fisheries or water, or for theprotection of flora and fauna. The derogationbrings with it a responsibility to monitor thestatus of these species.

Given these amendments, and the increasinginterest in the impact of corvids on their prey, itis timely to consider the patterns of populationchange among the corvids. Here we investigatechanges in population levels of 4 Britishcorvids, Jay, Magpie, Jackdaw and CarrionCrow, using data from the Common BirdsCensus (CBC).

METHODS

CBC data were analysed for the period1964�93.2,6 Census plots are chosen byobservers and are concentrated in the southand east of Britain.2 Observers make a series of

Bird Study (1995) 43, 28�37

© 1995 British Trust for Ornithology

Population trends of Jays, Magpies, Jackdaws andCarrion Crows in the United Kingdom

R.D. GREGORY* and J.H. MARCHANT British Trust for Ornithology,National Centre for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK

Population trends of Jay Garrulus glandarius, Magpie Pica pica, JackdawCorvus monedula and Carrion Crow Corvus corone were examined over theperiod 1964�93 using data from the Common Birds Census. Population growthhas been highest in the Magpie and Carrion Crow, although both these haveshown a reduction in the rate of growth from the late 1970s. Densities of all 4corvids have increased on farmland, whereas only Magpie and Carrion Crowhave increased in woodland. Population trends were associated with farmlandtype in all the corvids; the strongest increases were on mixed and grazing farmsand the smallest on arable farms. There were strong regional differences intrends within woodland and farmland. Our results are discussed in the contextof the long-term changes in corvid populations that have been reported since thestart of this century.

*Correspondence author.

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visits to their plot through the breeding seasonto record all the birds seen or heard. This infor-mation is used to identify clusters ofregistrations which are taken to representbreeding territories. Analysis of this kind forcorvids is, however, not straightforward. ForMagpie and Carrion Crow, large numbers ofbirds are generally taken to be non-breedersand excluded from territorial analysis. In thecase of Jackdaw, which often nests colonially,similar flocks are treated as breeding birds interritory analysis unless the plot lacks suitablenesting habitat. CBC plots comprise mostlyfarmland and woodland habitats with a small-er number of �special� plots from other habitattypes. Only the first 2 habitat types were usedin the analysis. Farmland plots must be morethan 40 ha in area and comprise any type oftilled, horticultural or grazing land, and do notinclude more than 10% woodland area withinthe plot boundary. Woodland plots must bemore than 10 ha and comprise any kind of nat-ural or semi-natural woodland, with woodlandusually making up over 90% of the plot area. In1993, there were 101 farmland and 109 wood-land plots censused. The mean areas of plotssurveyed have been relatively constant over theperiod we consider.2 Censuses with fewer than8 complete visits in a season, and others of poorquality, were excluded from the analyses.

Population trends were assessed usingchanges in the density of territories from1964�93, a period when counting methods andterritory analysis were highly standardized.Densities were estimated by dividing territorytotals by the plot area and are expressed as ter-ritories per square kilometre (= 100 ha).Species� �densities� should be seen as relativemeasures because the CBC does not providereliable estimates of absolute densities.7�9 Thesemeasures, used previously to study warblers(Sylviidae)10 and Magpie,3 may reduce some ofthe problems of non-independence of datapoints between years, and the effects of randomdrift, which may arise by chaining successiveindex values.11,12

Plots were categorized on the basis of habitatand regions as described below. These broadgroupings follow Gooch et al.3 and Marchant &Gregory.13 There were insufficient data for amore refined series of categories. Farmlandplots were categorized as either arable, mixedor grazing. Arable farms were those where all

fields were used for tillage, and grazing thosewhere no crops were grown and fields wereused for grazing or silage. Mixed farms includ-ed both tilled and grazing fields. Four regionswere defined on the basis of NUTS regions ofthe EC (Fig. 1).

Mean territory densities in 1993 were calcu-lated for each of the species and withincategories as appropriate. Note that values arenot strictly comparable among categoriesbecause estimates of density tend to decreasewith increasing plot area, especially where ter-ritory size is relatively large, and birddetectability varies with habitat type. Densitymeasures tend to be inflated on small plots dueto territories lying partly beyond the plotboundary. Nonetheless, these measures pro-vide a broad indication of habitat use.

Changes in population sizes were assessedusing generalized linear models within SAS.14

Two different approaches were used, the first toestimate the overall changes within habitats,and the second to explore differences amonghabitats and regions. Both approaches usedquadratic regression models to describe thecurvilinear patterns of population change. Inthe first, log10 (mean territory density acrossplots) was regressed against year. Populationchanges were estimated as the percentage dif-ference between the densities predicted fromthe regression equation at the start and end ofthe time span. Confidence limits about thesechanges were derived from the error about alinear regression slope. Limits were calculatedon a log scale and then back-transformed; thusthey are asymmetric about the change mea-sures. The second approach tested fordifferences among categories of the data, whichinvolved regressing log10 (plot territory density+ 1) against year, with year as a covariate, andhabitat or region as factors. Models were sim-plified sequentially by excluding non-significant interaction terms (at the 5% level).Since the regression models were quadratic,interaction terms were calculated from both theyear and year2 interaction terms. Significantinteraction indicates heterogeneity in popula-tion trends within the data. When there was nosuch interaction, a significant factor indicatesheterogeneity in densities across categories,and a significant year or year2 term, temporalchanges in densities. The unbalanced nature ofthe data precluded modelling densities as a

© 1995 British Trust for Ornithology, Bird Study, 43, 28�37

Corvid populations in the UK 29

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function of 2 class variables such as region andhabitat.

RESULTS

Within farmland, densities of Jay, Magpie,Jackdaw and Carrion Crow have increasedbetween 1964 and 1993 (Table 1, Fig. 2). Withinwoodland, only Magpie and Carrion Crowhave shown similar population gains, while Jayhas been stable and Jackdaw has declined(Table 1, Fig. 2). Magpie and Carrion Crowshowed a reduction in the rate of populationincrease from the late 1970s (Fig. 2). Linearregression of mean densities versus year in thefirst and second 15 years of the data set (com-bining farmland and woodland plots)

indicated that Magpie and Carrion Crowincreased by 268% and 140%, respectively,between 1964 and 1978, but by only 1% and10%, respectively, between 1979 and 1993. Thesame patterns emerge from analysis of farm-land and woodland separately. Densities in1993 give an indication of habitat preference ofthe corvids, and these were highest in wood-land (Table 1, Fig. 2). In addition, the densitiesof farmland Magpies and Carrion Crows havetended to be larger than those of Jackdaw andparticularly Jay (Table 1, Fig. 2). However, strictcomparison among habitats was not possiblebecause woodland plots were typically smallerthan those on farmland (see Methods).

Population trends of the Magpie andJackdaw differed between farmland and wood-

30 R.D. Gregory and J.H. Marchant

© 1995 British Trust for Ornithology, Bird Study, 43, 28�37

Figure 1. Regional divisions of the United Kingdom showing level 1 NUTS (Nomenclature of Territorial Unitsfor Statistics) regions of the EC. �North� comprises regions 71, 72, 78, 7A, 7B and the Isle of Man, �Midlands�regions 73 and 77, �South-east� regions 74 and 75, and �South-west� regions 76 and 79.

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land habitats (Table 2). Magpies have increasedin both habitats, although woodland densitiesappeared to peak in the mid 1970s and havesubsequently stabilized, whereas farmlandpopulations have shown a more consistent pat-tern of increase (Fig. 2). Jackdaws haveincreased slightly in farmland and declined inwoodland (Fig. 2). Trends for Jay and Carrion

Crow did not differ between farmland andwoodland (Table 2). However, their populationdensities differed between the two habitats andCarrion Crow has shown a significant curvilin-ear temporal trend (Table 2).

Population trends of all 4 corvids differedacross farmland habitats (Table 3, Fig. 3). Thepatterns of population change were variable

© 1995 British Trust for Ornithology, Bird Study, 43, 28�37

Corvid populations in the UK 31

Table 1. Changes in mean corvid densities on farmland and woodland Common Birds Census plots between1964 and 1993. D, mean territory density in territories per 10 ha in 1993, with its standard error; % change,estimated change in density between 1964 and 1993 with upper and lower 95% confidence limits (UCL and LCL)as predicted by the regression model; n is the range of numbers of census plots from which annual estimateswere drawn

Habitat Species D se % change LCL UCL n

Farmland Jay 0.8 0.2 57 18 108 43�83Magpie 4.6 0.5 241 197 201 43�82Jackdaw 3.1 0.7 91 41 158 41�83Carrion Crow 5.6 0.7 156 115 205 43�81

Woodland Jay 7.5 0.7 �2 �15 14 14�87Magpie 8.5 1.3 257 141 430 14�87Jackdaw 3.7 1.0 �58 �71 �38 12�85Carrion Crow 8.2 1.0 115 78 157 13�87

Figure 2. Population trends of Jay, Magpie, Jackdaw and Carrion Crow on farmland (solid line) and woodland(dashed line) Common Birds Census plots between 1964 and 1993. Mean corvid densities in territories per 10 hawere plotted on a logarithmic scale. Note that the scales on the y-axis are the same for Magpie and Carrion Crowbut differ for Jay and Jackdaw.

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(Fig. 3). Population gains tended to be smalleston arable farms and largest on mixed and graz-ing farms (Table 4, Fig. 3). On arable farms theconfidence intervals on the change estimatesinclude zero for all the corvids except Magpie.Linked to this, densities in 1993 were consis-tently lower on arable farm types whichsuggests that this was a relatively poor habitatfor corvids (Table 4).

Regional effects were examined within farm-land and woodland separately (Table 5). Withinfarmland, population trends differed amongregions for Jay, Magpie and Jackdaw. There wasno similar pattern for Carrion Crow, althoughdensities differed across regions and there wasa significant curvilinear time trend (Table 5).Within woodland, population trends ofMagpie, Jackdaw and Carrion Crow varied sig-nificantly across regions. Population trends ofwoodland Jays did not differ among regionsalthough densities differed significantly acrossregions (Table 5).

The regional variation in population trendsin both farmland and woodland was marked(Table 6). It should be noted that sample sizes

within many regions were small and the confi-dence limits extremely wide (Table 6). Thuspopulation changes within regions must beviewed with caution. Trends for farmland Jaysand Jackdaws were variable with the most pro-nounced increases in the south-west (Table 6).Population gains of Magpies and CarrionCrows in farmland have been relatively uni-form (Table 6). Regional changes withinwoodland differed in that population gains forJay and Jackdaw were highest in the north,while those for Magpie and Carrion Crow weremost rapid in the south-east (Table 6).

DISCUSSION

We have shown substantial increases in popu-lation levels of Magpie and Carrion Crowwithin the CBC data set over the last 30 years(Tables 1�6). The population growth of boththese corvids appears to have slowed since thelate 1970s (Fig. 2). Populations of Jay andJackdaw have been relatively stable with mod-est increases restricted to farmland habitats(Table 1). Although the overall pattern was of

32 R.D. Gregory and J.H. Marchant

© 1995 British Trust for Ornithology, Bird Study, 43, 28�37

Table 2. Variation in corvid trends and densities in farmland and woodland. F values are presented withsignificance levels and degrees of freedom. Log (territory density + 1) on Common Birds Census plots wasmodelled as a function of year, habitat and their interaction term (see text)

Effects

Species Year Year2 Habitat Year × habitat

Jay 0.9 (1,3857) 0.7 (1,3857) 3077*** (1,3857) nsMagpie 50.4*** (1,3833) 40.5*** (1,3833) 20.1*** (1,3833) 10.5*** (2,3833)Jackdaw 18.1*** (1,3798) 0.2 (1,3798) 1.6 (1,3798) 3.7* (2,3798)Carrion Crow 23.2*** (1,3789) 17.7*** (1,3789) 18.9*** (1,3789) ns

*P < 0.05, **P < 0.01, ***P < 0.001.

Table 3. Variation in corvid trends and densities across farmland habitats. The analysis follows that presented inTable 2. Farmland Common Birds Census plots comprise arable, mixed and grazing plot types (see text fordetails)

Effects

Species Year Year2 Farmland Year × farmland

Jay 0.01 (1,2057) 0 (1,2057) 6.6** (2,2057) 3.4** (4,2057)Magpie 1.4 (1,2037) 0.4 (1,2037) 1.1 (2,2037) 7.5*** (4,2037Jackdaw 0.1 (1,2024) 0.2 (1,2024) 1.9 (2,2024) 7.5*** (4,2024)Carrion Crow 7.1** (1,2012) 5.1* (1,2012) 0.7 (2,2012) 3.3* (4,2012)

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increase, there was variation across habitatsand across regions. The farmland regime had asignificant influence on population trends witharable farms providing a relatively poor habitatfor corvids (Table 3). It is not clear whether this

pattern was due to farming practices or someother factor associated with them; for example,levels of control or other environmental vari-ables. It is likely that regional variation (Table5) was driven by habitat differences which we

© 1995 British Trust for Ornithology, Bird Study, 43, 28�37

Corvid populations in the UK 33

Figure 3. Population trends of Jay, Magpie, Jackdaw and Carrion Crow on arable (solid line), mixed (dashed line)and grazing (dotted line) farmland Common Birds Census plots between 1964 and 1993. Mean corvid densitiesin territories per 10 ha were plotted on a logarithmic scale. Note that the scales on the y-axis are the same forMagpie and Carrion Crow but differ for Jay and Jackdaw.

Table 4. Changes in mean corvid densities on farmland Common Birds Census plots between 1964 and 1993,with respect to farmland habitat type (for details see Table 1)

Habitat Species D se % change LCL UCL n

Arable Jay 0.6 0.2 0 �60 149 7�22Magpie 1.8 0.5 55 17 107 7�20Jackdaw 1.5 0.8 17 �24 80 7�22Carrion Crow 1.8 0.5 20 �14 66 7�21

Mixed Jay 1.0 0.3 79 24 159 28�50Magpie 5.1 0.8 389 317 473 28�50Jackdaw 3.8 1.2 84 17 189 27�49Carrion Crow 6.5 1.0 198 153 250 28�50

Grazing Jay 0.7 0.2 126 16 340 8�27Magpie 6.2 1.2 161 120 211 8�26Jackdaw 3.5 1.5 165 73 307 7�25Carrion Crow 7.6 1.6 163 107 233 8�26

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were unable to measure. In the following dis-cussion we first consider each of the species inturn.

Jay

Jay populations have been stable in woodlandand shown a modest increase in farmland,although breeding densities in farmland werelow (Table 1, Fig. 2). Our data, and those fromthe New Atlas15 show Jay densities to be highestin the south of Britain.

Jays appear to have been increasing over thelast 50 years,1,2 although data from the NewAtlas show a small contraction in rangebetween 1970 and 1990.15 Population gains arethought to be due to reduced control by game-keepers and to the creation of newwoodlands.2,16,17

Some support for the control argument isprovided by the Game Conservancy Trust�sNational Game-Bag Census (NGBC) whichshows the numbers of Jays killed on a sampleof estates managed for shooting to have fallenbetween 1961 and 1989.18 However the majori-ty of Jays in this sample are killed in winter andthey may not be resident birds (S. Tapper, pers.comm.). A reduction in the numbers killedwould also be consistent with a general declinein population level, although we have shownno such decline. It should be noted that, like theCBC, the NGBC is not based upon a randomsample and that there is no control for differ-ences in keepering effort across estates.

Magpie

There were substantial increases in Magpiepopulations across habitats and regionsbetween 1964 and 1993 (Tables 1�6, Fig. 2).Population gains have been most restricted onarable farms (Table 4). Regional trends confirmthe patterns described by Gooch et al.,3 exceptthat we show a more pronounced increase inthe south-east. The difference probably relatesto the time periods studied (1966�86 by Goochet al.). Population gains in the south-east were afeature of this study (Table 6) and contrast withthe declines recorded in parts of easternEngland during the late 1950s and early 1960s.The latter were attributed to the removal ofhedgerows and the use of pesticides.1,16,19

Magpie densities are still low in much of east-ern England (Table 6).15

The historical increases in Magpies arebelieved to stem from reduced levels of controlthat began at the time of the First WorldWar.1,18,20 The spread of the breeding rangefrom rural farmland and woodland into subur-bia, which is only partly reflected in the CBCdata, has contributed to the high rate ofincrease.1,3,21 Magpie populations have beenstable in North America but have increased inmuch of Europe over the last 50 years.21 TheNGBC has shown a doubling in the number ofMagpies killed in Britain between 1961 and1989, reflecting the population growth.18,22 Injust 2 years up to 1991 there was a further dou-bling in the numbers killed22 and then a slightdecline in the next 2 years (S. Tapper, pers.

34 R.D. Gregory and J.H. Marchant

© 1995 British Trust for Ornithology, Bird Study, 43, 28�37

Table 5. Variation in corvid trends and densities across regions, within farmland and woodland Common BirdsCensus plots. The analysis follows that presented in Table 2. Regional divisions are presented in Fig. 1

Effects

Habitat Species Year Year2 Region Year × region

Farmland Jay 0.9 (1,2068) 0.6 (1,2068) 1.9 (3,2068) 3.0** (6,2068)Magpie 11.3*** (1,2048) 7.0** (1,2048) 5.3** (3,2048) 3.8*** (6,2048)Jackdaw 0.2 (1,2035) 0.7 (1,2035) 4.4* (3,2035) 9.6*** (6,2035)Carrion Crow 14.6 (1,2029) 10.7** (1,2029) 40.0*** (3,2029) ns

Woodland Jay 2.3 (1,1775) 2.1 (1,1775) 61.1*** (3,1775) nsMagpie 22.5*** (1,1767) 20.3*** (1,1767) 6.9*** (3,1767) 6.5*** (6,1767)Jackdaw 2.7 (1,1745) 2.6 (1,1745) 6.1*** (3,1745) 5.8*** (6,1745)Carrion Crow 5.6* (1,1746) 4.5* (1,1746) 7.7*** (3,1746) 6.5*** (6,1746)

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comm.). The recent changes are due to theintroduction of Larsen traps which represent aremarkably efficient method of trapping breed-ing Magpies (and Carrion Crows).22 Theirrecent introduction, however, means that theycannot be responsible for the slowing down ofpopulation growth of Magpie (and CarrionCrow) which began in the late 1970s (Fig. 2).Studies of Magpie populations in Sheffieldhave also shown a reduction in the rate ofincrease of territorial birds from around 1980,although the numbers of non-territorial birdshas continued to increase.21 The current effectsof control measures are difficult to judge butwith the advent of the Larsen trap it is conceiv-able that control measures might limit Magpienumbers.

Links are widely perceived between theincreases in Magpies and decreases in open-nesting songbirds. There is evidence from localstudies that Magpie predation can reduceBlackbird nesting success to the point whereproductivity is too low to maintain local popu-lations.4 On the national scale, however, Goochet al.3 found no decreases in nest success among15 species of songbirds that occur alongsideMagpies, and no evidence that increases inMagpie numbers were linked with declines insongbird populations.

Jackdaw

There has been a long-term increase in Jackdawpopulations during the present century and

© 1995 British Trust for Ornithology, Bird Study, 43, 28�37

Corvid populations in the UK 35

Table 6. Changes in mean corvid densities on farmland and woodland Common Birds Census plots between1964 and 1993, with respect to regions (for details see Table l)

Region Habitat Species D se % change LCL UCL n

North Farmland Jay 0.3 0.2 �11 �57 83 6�24Magpie 3.4 0.9 271 125 510 6�23Jackdaw 2.9 1.3 395 198 720 5�23Carrion Crow 3.8 0.8 111 64 172 6�22

Woodland Jay 5.3 1.6 102 34 204 3�23Magpie 5.2 1.8 355 87 1006 3�23Jackdaw 4.0 2.6 529 19 3223 2�22Carrion Crow 8.1 2.4 106 18 260 3�23

Midlands Farmland Jay 0.1 0.1 9 �48 l23 8�16Magpie 4.6 1.4 285 228 510 7�16Jackdaw 6.0 3.5 105 4 303 8�16Carrion Crow 6.7 1.9 287 197 403 7�16

Woodland Jay 4.8 1.9 �54 �70 �30 1�9Magpie 8.0 5.0 35 �47 246 1�9Jackdaw 1.5 1.0 �52 �89 100 1�9Carrion Crow 5.8 1.9 35 �43 216 1�9

South-east Farmland Jay 1.0 0.2 66 19 130 18�38Magpie 4.9 0.8 298 228 383 18�36Jackdaw 1.6 0.6 �34 �55 �3 18�38Carrion Crow 4.9 0.9 153 108 208 18�38

Woodland Jay 8.6 1.1 �4 �17 11 8�47Magpie 9.8 1.4 426 287 616 8�47Jackdaw 5.2 1.6 �62 �77 �40 7�47Carrion Crow 8.7 1.4 186 131 254 7�47

South-west Farmland Jay 1.8 0.5 213 74 461 7�19Magpie 6.3 1.7 249 165 360 7�19Jackdaw 6.0 2.1 1400 703 2700 7�19Carrion Crow 10.1 2.9 185 116 278 7�19

Woodland Jay 9.1 1.2 20 �12 63 1�20Magpie 10.2 4.0 53 �10 161 1�20Jackdaw 1.8 1.1 �89 �98 �50 1�19Carrion Crow 8.6 2.0 38 �9 108 1�20

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this trend has been linked with changes in pat-terns of cultivation.1,20 Our results showJackdaw populations to have declined inwoodland but increased slightly in farmlandover the last 30 years (Tables 1 & 2, Fig. 2). Thepattern in woodland differs from that present-ed by Marchant et al.2, who describedpopulation increases using �chained� CBCindices (1976�88).2 This discrepancy probablyrelates to the different study periods. It shouldbe stressed that the aggregated and semi-colo-nial nesting habit of this species makes censuswork problematic; in addition, their popula-tions are concentrated in the west of Britain andoften in habitats outside the scope of the CBC.15

Distributional data show the range of theJackdaw to have contracted between 1970 and1990,15 and the NGBC shows the number ofJackdaws killed to have fallen between 1961and 1989.18

Jackdaw trends varied across habitats andregions (Tables 2�6). Population gains havebeen most pronounced on grazing farms and inthe north and south-west where such farmspredominate. Woodland Jackdaws increasedonly in the north. Previous studies have raisedthe importance of grassland as a feeding areafor Jackdaws and also the availability of suit-able nest sites.20

Carrion Crow

Carrion Crows have increased considerablyover the last 30 years (Tables 1�6; Fig. 2).Changes in Carrion Crow populations closelymirrored those of the Magpie, althoughincreases tended to be slightly smaller. LikeMagpies, Carrion Crows have shown a generalreduction in population growth since the late1970s. O�Connor & Shrubb20 suggested that thegeneral increase in density of sheep in uplandareas, and consequent increase in carrion, maybe responsible for the expansion of CarrionCrow and Magpie populations. However, theyshowed that Carrion Crows on CBC plots wereincreasing in counties dominated by cerealsand tillage and were stable in sheep-rearingregions. We find population gains on bothmixed and grazing CBC plots and only a smallincrease on arable plots. Comparison is difficultbecause O�Connor & Shrubb assigned all CBCplots in a county to the predominant farmlandtype in that county, and their period of study

(1962�84) was different from that consideredhere. The New Atlas shows population densitiesto be low in much of eastern England.15

The Carrion Crow has increased greatly overthe last 50 years, primarily as a consequence ofdecreased control, and has extended its rangeinto suburban and urban areas.1,18,20 The NGBChas shown no overall change in the number ofCarrion Crows killed since 1961 but a smallincrease from 1988 (S. Tapper, pers. comm.).18,22

This is surprising given the general populationincrease and perhaps suggests a reduction ineffort directed to the control of this species.18

Despite the introduction of Larsen traps, whichare equally efficient at capturing territorialCarrion Crows and Magpies, there has beenonly a small upturn in recent Carrion Crow bagreturns.22 As for the Magpie, the role of controlmeasures at a national scale is unclear.

Corvids and songbirds

Recent analyses have shown farmland birds,particularly seed-eating species, to be decreas-ing in numbers and contracting their rangescompared with their woodland counter-parts.13,15,23 One hypothesis to explain thesechanges is that they are due to predation pres-sure from increasing numbers of corvids onfarmland. Population gains among corvidswere, however, lower on arable compared toother farm types (Table 3), yet it was withinintensive farmland that population declines ofseed-eaters were most pronounced.13 If corvidswere the sole driving force behind populationdeclines we would expect them to be at highestdensities or increasing most in these arableareas. Our data suggest that arable farms pro-vide a relatively poor habitat for corvids. It isclear that the population dynamics of bothcorvids and songbirds are likely to be complex.Further work is required to assess the factorslimiting population growth of corvids, and thepotential links between population levels ofcorvids and songbirds.

ACKNOWLEDGEMENTS

We thank the many hundreds of volunteerbirdwatchers who have contributed to the CBCover the last 30 years. Thanks are also due toDawn Balmer for help with data checking, to

36 R.D. Gregory and J.H. Marchant

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Dr S.R. Baillie, Dr J.J.D. Greenwood and Dr S.Tapper for statistical advice and discussion,and to Dr R.E. Green and Prof. T.R. Birkheadfor helpful comments on an earlier draft.

The Common Birds Census forms part of theBTO�s Integrated Population MonitoringProgramme carried out under contract from theJoint Nature Conservation Committee onbehalf of English Nature, the CountrysideCouncil for Wales and Scottish NaturalHeritage, and under a contract from theDepartment of the Environment for NorthernIreland. This work is based upon a contractfrom the Joint Nature Conservation Committeeto assess the status of a number of pest speciesin the United Kingdom. We are grateful toRobin Groombridge and Susan Davies for theirhelp and advice during the course of this pro-ject.

REFERENCES

1. Parslow, J.L.F. (1973) Breeding Birds of Britain andIreland. Poyser, Berkhamsted.

2. Marchant, J.H., Hudson, R., Carter, S.P. & Whit-tington, P.A. (1990) Population Trends in BritishBreeding Birds. BTO/NCC, Tring.

3. Gooch, S., Baillie, S. & Birkhead, T.R. (1991) Theimpact of Magpies Pica pica on songbird popula-tions. Retrospective investigation of trends inpopulation density and breeding success. J. Appl.Ecol., 28, 1068�1086.

4. Groom, D.W. (1993) Magpie Pica pica predation onBlackbird Turdus merula nests in urban areas. BirdStudy, 40, 55�62.

5. Potts, G.R. (1986) The Partridge: Pesticides, Preda-tion and Conservation. Collins, London.

6. Marchant, J.H. (1983) BTO Common Birds CensusInstructions. BTO, Tring.

7. Dawson, D.G. (1981) The usefulness of absolute(�census�) and relative (�sampling� or �index�) mea-sures of abundance. Stud. Avian Biol., 6, 554�558.

8. Verner, J. (1985) Assessment of counting tech-niques. In Current Ornithology Vol 2, (ed. R.F.Johnston), pp. 247�302. Plenum Press, New York.

9. Bibby, C.J., Burgess, N.D. & Hill, D.A. (1992) Bird

Census Techniques. Academic Press, London.10. Lack, P. (1989) Overall and regional trends in war-

bler populations of British farmland over 25years. Ann. Zool. Fennici, 26, 219�225.

11. Geissler, P.H. & Noon, B.R. (1981) Estimates ofavian population trends from the North AmericanBreeding Bird Survey. Stud. Avian Biol., 6, 45�61.

12. Moss, D. (1985) Some statistical checks on theBTO Common Birds Census index � 20 years on.In Bird Census and Atlas Studies, (eds K. Taylor, R.J.Fuller & P.C. Lack), pp. 175�179. BTO, Tring.

13. Marchant, J.H. & Gregory, R.D. (1994) Recent pop-ulation changes among seed-eating passerines inthe United Kingdom. In Bird Numbers 1992. Distri-bution, Monitoring and Ecological Aspects. Proc 12thInt. Conf. IBCC and EOAC, Noordwijkerhout, TheNetherlands (eds E.J.M Hagemijer & T.J. Verstrael)pp. 87�95. Statistics Netherlands Voorburg/Heerlen & Sovon, Beck Ubergen.

14. SAS (1985) SAS User�s Guide: Statistics. Version 5.SAS Institute Inc, Cary, NC.

15. Gibbons, D.W., Reid, J.B. & Chapman, R.A. (1993)The New Atlas of Breeding Birds in Britain and Ire-land: 1988�1991. Poyser, London.

16. Prestt, I. (1965) An enquiry into the recent breed-ing status of some of the smaller birds of prey andcrows in Britain. Bird Study, 12, 196�221.

17. Sharrock, J.T.R. (1976) The Atlas of Breeding Birds inBritain and Ireland. Poyser, Berkhamsted.

18. Tapper, S. (1992) Game Heritage: an EcologicalReview from Shooting and Gamekeeping Records.Game Conservancy, Fordingbridge.

19. Cooke, A.S. (1979) Population declines of theMagpie Pica pica in Huntingdonshire and otherparts of eastern England. Biol. Conserv., 15,317�324.

20. O�Connor, R.J. & Shrubb, M. (1986) Farming andBirds. Cambridge University Press, Cambridge.

21. Birkhead, T.R. (1991) The Magpies: the Ecology andBehaviour of Black-billed and Yellow-billed Magpies.Poyser, London.

22. Tapper, S. & France, J. (1992) The National GameBag Census 1991. Game Conserv. Annu. Rev., 23,38�40.

23. Fuller, R.J., Gregory, R.D., Gibbons, D.W.,Marchant, J.H., Wilson, J.D., Baillie, S.R. & Carter,N. (1995) Population declines and range contrac-tions among lowland farmland birds in Britain.Conserv. Biol. 9.

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