Download 4b.1 Introduction

Appendix 4b.
4b.1
The RSPB’s Bird Aid Project: the effects of supplementary winter
seed food on breeding populations of farmland birds
Introduction
Many seed-eating bird species that specialise in farmland habitats have shown consistent population
declines in the UK since 1970, when compared to more generalist species (Fuller et al. 1995;
Siriwardena et al. 1998a). There is a wealth of evidence suggesting that major changes in agricultural
practices have been the main driver for these declines (Campbell et al. 1997, Siriwardena et al. 1998a,
Chamberlain et al., 2000, Donald et al., 2001, Anderson et al. 2001). A major demographic
mechanism for these declines is likely to be greater annual mortality rates, perhaps resulting from
reductions in winter food resources over recent decades (Siriwardena et al. 1998b, Peach et al. 1999,
Siriwardena et al. 2000, Bradbury & Stoate 2000). Reduced annual survival due to lack of winter seed
food has been linked with breeding population decline in rural house sparrows (Hole et al. 2002).
Intensive studies on corn buntings have shown that they may be sensitive to a wide range of recent
changes in agriculture, but particularly the reduced availability of food during the winter and early
spring (Brickle et al. 2000, Donald & Aebischer 1997, Donald & Evans 1994). Breeding
yellowhammers (a sedentary species) are associated with arable and mixed landscapes that contain
stubbles in winter rather than pastoral landscapes where stubbles are absent (Kyrkos et al. 1998).
With 34% of the UK countryside dedicated to production crops, reversing the declines of these
species depends, in the long term, on major changes to agricultural policy. In the interim, considerable
research effort is being focused on investigating ways to ensure that populations that are perceived to
be isolated and vulnerable are still extant when policy change occurs.
In this paper, we investigate the effectiveness of providing supplementary winter seed food in
producing more positive local population trends in breeding bird numbers. The Royal Society for the
Protection of Birds’ (RSPB) Bird Aid project was established in 2000, initially as a direct intervention
conservation measure to provide supplementary seed food in winter and, hence, to attempt to maintain
local populations of seed eating species until new policy measures were able to provide seed-rich
habitats on a sufficiently wide scale. Seed food was provided by supplementary feeding and through
manipulation of existing arable rotations (leaving sacrificial crops, or growing wildbird cover). The
Bird Aid project was targeted towards three species; tree sparrow Passer montanus, yellowhammer
Emberiza citrinella and corn bunting E. calandra. All three have shown large UK population declines
since 1970 (Gregory et al. 2002) and are now localised across much of the UK and/or are in danger of
becoming extinct in some regions/counties. By comparing changes in local breeding populations
surrounding feeding sites with those on unfed control sites, the project also provided the opportunity
for an experimental test of the extent to which the availability of winter seed food represented a
limiting factor for these three species.
Emergency intervention methods have been used in the past to help threatened populations on a local
level; in 1990, when research demonstrated the reliance of cirl buntings Emberiza cirlus on weedy
stubbles as a winter food resource (Evans 1997), provision of these habitats became one main focus of
targeted conservation action which has resulted in subsequent population increase (Peach et al.
2001). However, until now, no such wide-scale feeding experiment has been conducted on European
farmland.
4b.2
Methods
Site selection and supplementary food provision
Site selection and provision of supplementary food for the Bird Aid project is described in full by
Siriwardena & Stevens (2004). All feeding sites were located on bare areas of relatively sparse
vegetation where the seed would be visible to birds, yet were generally situated close to some form of
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low cover, as many farmland bird species use cover as a refuge to reduce the risk of predation (Lima
& Dill 1990, Robinson & Sutherland 1997). Where possible, locations were chosen so as to reduce the
chance of other disturbance events, for instance from farm management work, to a minimum.
One hundred and nine farmland study sites across the UK were selected. Sites were chosen based on
current or recent local records of at least 20 individuals of one, or more, of the three target species.
Sites were stratified by target species (tree sparrow, yellowhammer and corn bunting) and region,
each site comprising one tetrad (a 2km x 2km square), centred either on the supplementary feeding
site or a comparable location on control sites. The chosen target species were considered appropriate
for this study because they are resident (Wernham et al. 2002), are all winter-flocking and are also all
readily surveyed in breeding season. Within regions, sites were randomly allocated to ‘fed’ or
‘control’ treatments. Fifty-two supplementary feeding sites were established, the other 57 sites being
control sites.
Data collection – bird counts
Breeding season surveys were carried out in 2000 (a pre-feeding baseline year), 2002 and 2003. No
surveys were conducted in 2001 due to restrictions arising from the foot-and-mouth outbreak. Each
tetrad was visited twice, once in the early part of the breeding season (April – May), and once in the
later part of the breeding season (June – July). Surveys were timed to start at c.08:00 and to finish by
c.13:00. All agricultural habitats in the tetrad were surveyed. Transects were walked at an even pace,
(with stops to listen for songs and calls at about every 250m for a couple of minutes) using access
points and paths as much as possible to get within 250m of all points within the 2 × 2km tetrad.
Exactly the same route was walked on subsequent visits to a site but with a randomised start point and
direction to avoid bias due to time of day. Constant rain, strong wind (>force 4) and days of poor
visibility were avoided (Robbins 1981). A sub-set of 23 sites in England (12 fed, 11 control) were
surveyed 6 times between 1 April and 31 July in 2002 and 2003 to obtain more accurate estimates of
breeding population size. In addition, each feeding site was visited twice in each winter of the
experiment (2000/01-2002/03), once in November or December and once in January or February.
During each visit, observations of the feeding site were made from a clear vantage point, avoiding
disturbance to the birds, for a 60-minute period between 10:00 and 12:00 hrs. During the watch, the
maximum number of birds of each species using the site was recorded every 10 minutes.
Analyses
Since Bird Aid was conceived to target supplementary feeding at key species, all sites were within the
known ranges of those species and as such, site selection was non-random, being located where
‘good’ numbers (20+ individuals) of the target species were present. Data for sites were not included
if the site fell in an area outside the known range for the species, or if the species was not recorded on
any monitoring visit in any season or year of the project. Data selection was therefore based on sites
where the species was known to occur and could therefore, in theory, respond to the supplementary
food treatment. For all three breeding season surveys of the project, 47 sites thus provided Tree
Sparrow data (25 fed, 22 control), 96 for Yellowhammer (46 fed, 50 control) and 46 for Corn Bunting
(22 fed, 24 control).
Analysis tested whether or not supplementary feeding increased breeding populations of the three
target species between 2000 and 2003 on experimental sites relative to control sites. In order to ensure
that analyses did not include effects that were a result of changes in breeding productivity, counts of
juvenile birds were excluded from the analyses. For tree sparrows, we used the maximum count of
adult birds on each site from visits that fell between 15th April and 15 th June in each year. Selecting
counts between these dates allowed us to avoid counts of birds in late winter flocks, whilst at the same
time minimising the chance of including juvenile birds in the counts. For yellowhammers we used the
maximum count of adult males at each site in each year. For corn buntings, a sexually monomorphic
species, the maximum count of territorial singing males was used for analyses. As open farmland
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formed the majority of all surveyed tetrads, it was considered that detectability was equal across all
sites.
Analysis was based on the change in the count of each species on each site between years. Variation
in counts for each species was assessed in generalized linear mixed models (GzLMMs) with Poisson
error structure, a log link function and controlling for overdispersion. The GzLMMs were fitted using
the macro GLIMMIX supplied with SAS 8.02 (Littell et al. 2002). Site identity was included as a
random effect to account for average differences in counts between sites that occurred irrespective of
feeding site location. The model specified was:
count = site + year + treatment + year×treatment
where treatment is a 2-level factor (0=control sites, 1=fed sites), and site is a multilevel factor
equivalent to the number of sites in the study (‘All Sites’ models). Separate models were run with
year as a 3-level categorical variable (2000, 2002 & 2003), and as a continuous variable (effectively to
smooth out post-treatment annual variation). Differences in breeding population trajectory between
treatments were determined by testing for significant year  treatment interaction terms. For the data
sets from intensively surveyed sub-set of sites in 2002 and 2003, no equivalent baseline (pre-feeding)
data existed for 2000. Therefore only data from 2002 and 2003 were included in these models
(‘Intensive Sites’ models), with year coded as a two-level factor, and significance of the main
treatment term assessed in addition to the year  treatment interaction term.
In order to check whether improved efficacy of feeding sites improved the data, models were also run
using a subset of the data from experimental sites where the target species had been observed and
therefore confirmed to have used the supplementary food patch in one or more of the three winters
within the study period (‘Effective Sites’ models).
The effect of the level of use of the feeding stations made by the target species in winter on changes in
breeding population was also investigated. Breeding season counts of each target species in tetrads
surrounding feeding sites only, were analysed in relation to the numbers of that target species
observed using the feeding sites. For each of the three winters during the experimental period, the
maximum number of individuals recorded using the supplementary food across the two winter survey
visits was calculated and the mean of these figures across the three winters used as a measure of
feeding site use. Breeding populations in tetrads surrounding each feeding site at the end of the
experimental period (2003) were then analysed in relation to this winter site measures, controlling for
initial breeding populations (2000). Generalised Linear Models (GzLMs) of the form: Breeding
season tetrad count in 2003 = Breeding season tetrad count in 2000 + winter site use, with poisson
error structure, log-link function and corrected for overdispersion were run using the GENMOD
procedure within SAS 8.02 (‘Site Use’ models). Significance of the winter site use measure was
assessed using likelihood-ratio tests.
4b.3
Results
Despite all three target species being recorded using many of the feeding sites provided, no significant
effect of the feeding treatment on breeding populations was detected over the course of the whole
study (2000-2003) for any of the three key species (Tables 4a.1 and 4a.2).
The trends in the data for local breeding population changes between 2000 and 2003 for all 3 species
are at least in the expected direction (i.e. more positive population trends on fed sites compared to
controls), but the differences in trends between fed and control sites are relatively small and not
statistically significant (Figure 4a.1).
The ‘intensive sites’ models also indicated no positive effects of the feeding treatment, indeed tree
sparrow showed a significant negative effect of feeding treatment on the population trajectory from
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2002 – 2003 (Table 4a.3). Results from these models should be treated with caution however, due to
small samples sizes and lack of pre-treatment data from 2000. Changes in breeding population
between 2002 and 2003 are not associated with any change in the feeding treatment.
‘Site use’ models showed a significant positive effect of the level of winter feeding site use over the
experimental period on the breeding population trajectory of yellowhammers, but not tree sparrow or
corn bunting (Table 4a.4).
4b.4
Discussion
Although Bird Aid feeding sites proved successful in attracting many farmland birds (Siriwardena &
Stevens, 2004), the overall value of supplementary feeding (as conservation management aimed at
removing a population limiting factor) depends on achieving positive responses among breeding
populations. The only indication that supplementary food provision was related to more positive
population change came from the ‘Site Use’ model for yellowhammer. However, as this analysis is
not ‘experimental’ in nature, i.e. it only considers data from feeding sites without controls, it is
possible that some other factor resulted in population change and that greater use of a feeding site is
simply a reflection of an increased local population, without any causal relationship.
It is possible that there was no effect of supplementary winter seed food in the form tested during this
experiment is on the survival rates of seed-eating farmland songbirds. Another factor, e.g. availability
of nest sites or suitable habitats for obtaining chick food, may have limited the breeding populations.
However, there are also several other possible explanations for the results obtained here:
The quantity of food put out was not sufficient
There may be an effect of supplementary winter seed food on the survival rates of seed-eating
farmland songbirds, but the quantity of food provided during the experiment was insignificant and/or
most of the local seed-eating birds could not utilise this resource for some reason (e.g. because they
never found the feeding site). The feeding at Bird Aid sites was patchy on many sites in the first
winter of the project. Further advice and instruction to the landowners very much improved the
consistency of feeding by the second winter, but some sites remained unreliable in the supply of seed
available to birds. This was accounted for to some extent in the analyses by testing a sub-set of the
data from sites where we knew that the target species had been attracted to the supplementary winter
food.
Breeding season responses may not have been detected
It is possible that the provision of supplementary winter seed food at the scale used in this study does
increase the over-winter survival rates of seed-eating farmland songbirds, and does result in more
breeding birds in subsequent seasons, but the fieldwork carried out was unable to detect this. This
could happen if:
(i)
The projects were not run for long enough to detect a large enough effect on the local
breeding populations. It may be that there is a small incremental increase in breeding
populations each year, as a result of the feeding treatment. This possibility is supported by
the results showing more positive (but non-significant) trends on feeding sites compared
to control sites.
(ii)
If birds move significant distances between their breeding territories and winter feeding
locations, then any positive effect of a single seed feeding station on the survival rates of
individual birds using that site may be effectively ‘diluted’ over a wide area of breeding
habitat during the summer. If the breeding season ‘catchment area’ of an individual
winter-feeding station were much larger than the surrounding tetrad, then surveys within
the tetrad would only detect a proportion of the total population increase due to increased
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survival rates. Some increase in the local tetrad population would still be expected
however, unless the quality of the breeding season habitats within the tetrad area were
systematically poorer than those of the surrounding area. There is no a priori reason why
this should be the case, and the number of study sites involved in the experiment should
prevent the possibility that this occurred by chance, on average, across all sites.
Increasing the area surveyed in the breeding season would increase the probability that a
greater proportion of the birds visiting the feeding station would remain within the
surveyed area during the breeding season. However, increasing the surveyed area around
feeding sites to a square of 4x4 km, rather than 2x2 km, would increase the area surveyed
four-fold which was not achievable with the project resources available.
Although we believe that tree sparrows, yellowhammers and corn buntings are essentially
sedentary (i.e. non–migratory) within the UK, the average distances that these and other
farmland bird species move between summer breeding territories and winter foraging
patches is little known (Calladine et al., in press). Further work is needed to estimate the
scale of these movements.
By comparing the maximum counts of birds recorded in winter on the feeding stations
with the maximum number of breeding birds recorded in the surrounding tetrad, it is
strongly suspected that some feeding sites attracted many more birds in winter than were
present in the surrounding tetrad in summer. Conversely, it is also plausible that other
sites failed to attract the majority of birds breeding in the surrounding tetrad, these
forming wintering flocks elsewhere and outside the tetrad. Both sources of variation will
weaken the potential relationship between feeding stations and population increases in the
area surrounding them.
(iii)
If there is considerable variation in the quality of the areas surrounding our feeding
stations/control sites in terms of their suitability as breeding habitats, this may have a
great influence on the ability of local breeding populations to increase or not, even if
supplementary food does increase winter survival rates.
(iv)
The initial selection of sites for the Bird Aid project concentrated on areas where ‘good’
populations (20+ individuals) of the target species had been recorded recently (either in
summer or winter). If the main factor influencing the continued presence of birds in these
areas had been the availability of winter seed food, then the feeding treatment may have
had relatively little additional effect. In this case, both the fed and control sites may have
been ‘better’ areas for the target species than much of the surrounding landscape. The
implication is that the control sites (as well as the fed sites) were already suitable habitat
for the target species, with adequate winter food resources already present, regardless of
the extra supplementary food provided. This was a conclusion of a similar, but much
smaller scale, winter feeding experiment on farmland house sparrows in Oxfordshire
(Hole et al. 2002), where four study sites, all with good existing populations of house
sparrows (30+ individuals per site) were chosen for methodological reasons. Only one
site showed genetic and demographic evidence of recent and ongoing population decline,
and only this site showed a positive response to the provision of supplementary seed food
(an increase in over-winter survival rates).
Food shortages are only critical during cold winters
It could be that the provision of supplementary seed food only benefits birds during cold winters.
Increased mortality will result when frozen ground and/or snow cover prevents access to seed food by
birds already hard-pressed to find adequate food resources. There was been very little cold weather
(mean daily temperatures below 0C) and only short periods of snow cover in the UK during the
period of these two projects.
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Despite providing no robust consistent evidence that the provision of supplementary seed food results
in increased local breeding populations, Bird Aid suggests that simple, relatively small-scale feeding
sites can attract good numbers of granivorous farmland birds during winter (Siriwardena & Stevens,
2004). There is also evidence that farmers enjoyed participating in the projects and would like to
continue feeding birds through the winter (RSPB, unpublished data); evidence that in terms of
popularity at least, supplementary feeding of this nature represents a realistic option at least in the
short term. Conservation value may be gained by targeting supplementary feeding sites in regions
where key farmland species are rare, but measuring any positive response to such actions remains
difficult. Future analyses of Bird Aid data will focus on ways in which feeding site usage is affected
by the local landscape (both in terms of winter food resources and breeding habitat availability) and
the distribution and abundance of birds within it.
4b.5
Acknowledgments
The Bird Aid project was funded by RSPB, with additional funding for increased survey frequency on
a sub-set of sites from Defra. Steve Dodd and Richard Winspear provide invaluable logistic assistance
and project management. Richard Bradbury, Dave Buckingham, Will Peach and Gavin Siriwardena
provided advice on analysis. Bill Sutherland, Neil Calbrade and Steve Dodd have provided valuable
assistance. We are very grateful to all who helped with fieldwork, including; Dominic Coath, Michael
Copplestone, Claire Devereux, Larry Griffin, Frazer Macfarlane, Kevin Mawhinney, Trevor Smith,
Roger Taylor, Emily Woodfield & Andy Wraithmell. The cooperation of many landowners made our
studies possible and we are grateful to them all.
4b.6
References
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intensification on wild bird populations in the UK. RSPB Research Report No. 3. RSPB, Sandy, UK.
Bradbury, R.B. & Stoate, C. (2000) The ecology of yellowhammers Emberiza citrinella on lowland
farmland. pp 165-172 in: Aebischer, N J, Evans, A, Grice, P & Vickery, J (Eds) Ecology and
Conservation of Lowland farmland birds. Proceedings of the 1999 British Ornithologists’ Union
Spring Conference, Southampton. Tring, British Ornithologists’ Union.
Brickle, N.W., Harper, D.G.C., Aebischer, N.J., Cokayne, S.H., 2000. Effects of agricultural
intensification on the breeding success of corn buntings. Journal of Applied Ecology 37: 742-755.
Calladine, J., Robertson, D. & Wernham, C. in press. The ranging behaviour of some granivorous
passerines on farmland in winter determined by mark-recapture ringing and by radio-telemetry. Ibis.
Campbell, L.H., Avery, M.I., Donald, P.F., Evans, A.D., Green, R.E., Wilson, J.D., 1997. A review of
the indirect effects of pesticides on birds. JNCC report no. 227, Joint Nature Conservation Committee,
Peterborough, UK.
Chamberlain, D.E., Fuller, R.J., Bunce, R.G.H., Duckworth, J.C., Shrubb, M., 2000. Changes in the
abundance of farmland birds in relation to the timing of agricultural intensification in England and
Wales. Journal of Applied Ecology 37: 771-788.
Donald, P.F. & Aebischer, N.J. (Eds) 1997 The ecology and conservation of corn buntings Miliaria
calandra. UK Nature Conservation No. 13. Peterborough: Joint Nature Conservation Committee.
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Pienkowski, M.W. (Eds.), Farming and Birds in Europe: the Common Agricultural Policy and its
implications for Bird Conservation, Academic Press, London, pp. 331-357.
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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 contractions among lowland farmland birds in Britain.
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Krebs, J.R. 2002. Widespread local house-sparrow extinctions. Nature 418: 931-932.
Kyrkos, A., Wilson, J.D. & Fuller, R.J. (1998) Farmland habitat change and abundance of
Yellowhammers Emberiza citrinella: an analysis of Common Birds Census data. Bird Study, 45, 232246.
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prospectus. Canadian Journal of Zoology 68: 619-640.
Littell, R.C., Milliken, G.A., Stroup, W.W., Wolfinger, R.D., 2002. SAS® System for Mixed Models.
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Peach, W.J., Siriwardena, G.M. & Gregory, R.D. 1999. Long-term changes in over-winter survival
rates explain the decline of reed buntings Emberiza schoeniclus in Britain. J. Appl. Ecol. 36: 798-811.
Peach, W.J., Lovett, L.J., Wotton, S.R., Jeffs, C., 2001. Countryside stewardship delivers cirl buntings
(Emberiza cirlus) in Devon, UK. Biological Conservation 101: 361-373.
Robbins, C.S. 1981. Bird activity related to weather. Studies Avian Biol. 6: 301-310.
Robinson, R.A. & Sutherland, W.J. 1997 The feeding ecology of seed-eating birds on farmland in
winter. In Donald, P.F. & Aebischer, N.J. (Eds) The Ecology and Conservation of Corn Buntings
Miliaria calandra: 27-41. Peterborough: JNCC.
SAS Institute, Inc., 2000. SASOnlineDoc®, Version 8. Cary, NC: SAS Institute Inc.
Siriwardena, G.M., Baillie, S.R., Buckland, S.T., Fewster, R.M., Marchant, J.H., Wilson J.D., 1998a.
Trends in the abundance of farmland birds: a quantitative comparison of smoothed Common Birds
Census indices. Journal of Applied Ecology 35: 24-43.
Siriwardena, G.M., Baillie, S.R. & Wilson, J.D., 1998b. Variation in the survival rates of some
British passerines with respect to their population trends on farmland. Bird Study 45: 276-292.
Siriwardena, G.M., Baillie, S.R., Crick, H.Q.P., Wilson, J.D. & Gates S. 2000. The demography of
lowland farmland birds. In Aebischer, N.J., Evans, A.D., Grice, P.V. & Vickery, J.A. (Eds) Ecology
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GS/Winter Feeding II/Appendices CSG15/Appendix 4b
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Table 4a.1
Outcome of the GzLMM’s investigating the effect of supplementary winter feeding
on breeding populations at Bird Aid sites: ‘All Sites’ models. Significant results are
shown in bold.
Year as 3-level factor
Yr
F
Tree sparrow
Year as continuous variable
Trt
P
F
P
Yr*Trt
Yr
F
F
P
Trt
P
F
Yr*Trt
P
F
P
3.01 0.055 1.70 0.200 0.59 0.558 1.44 0.234 0.13 0.716 0.88 0.352
Yellowhammer 6.23 0.002 0.35 0.558 0.64 0.530 0.27 0.605 0.00 0.995 0.93 0.337
Corn bunting
Table 4a.2
0.48 0.620 4.07 0.052 0.50 0.607 0.83 0.440 0.68 0.415 0.33 0.718
Outcome of the GzLMM’s investigating the effect of supplementary winter feeding
on breeding populations at Bird Aid sites: ‘Effective sites’ models. Significant results
are shown in bold.
Year as 3-level factor
Year
F
Tree sparrow
P
Year as continuous variable
Treatment
Yr*Trt
Year
F
F
F
P
P
P
Treatment
Yr*Trt
F
F
P
P
1.80 0.173 1.94 0.174 0.89 0.414 0.97 0.327 0.08 0.785 1.60 0.211
Yellowhammer 5.43 0.005 0.68 0.414 0.98 0.379 0.03 0.859 0.00 0.962 1.51 0.220
Corn bunting
0.83 0.440 0.68 0.415 0.33 0.718 1.24 0.269 0.69 0.411 0.02 0.890
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Table 4a.3
Results of the GzLMM’s investigating the effect of supplementary winter feeding on
breeding populations at Bird Aid sites: ‘Intensive sites’ models. Significant results are
shown in bold ‘-’ Sign in brackets after significant result indicates more negative
population trend on experimental sites relative to control sites.
Year
F
Tree sparrow
P
Treatment
Yr*Trt
F
F
P
P
0.37 0.547 0.00 0.973 6.60 0.017 (-)
Yellowhammer 1.51 0.238 1.78 0.201 0.03 0.862
Corn bunting
Table 4a.4
2.25 0.152 0.24 0.631 0.54 0.474
Results of GzLM’s investigating the effect of the level of winter feeding site use on
breeding population change at Bird Aid sites (‘Site Use’ models). Significant results
are shown in bold, ‘+’ sign in brackets after significant result indicates positive effect
winter feeding site use on population trend.
Winter Site Use Measure
Tree sparrow
F
P
0.05
0.817
Yellowhammer 5.28
0.027 (+)
Corn bunting
0.385
0.79
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Figure 4a.1
Back-transformed parameter estimates of breeding bird counts at experimental
feeding (▲) and control (■) sites, for (a) tree sparrow, (b) yellowhammer, and (c)
corn bunting.
(a)
10
Parameter estimate
8
6
4
2
0
2000
2001
2002
2003
2002
2003
2002
2003
Year
Parameter estimate
(b)
9
8
7
6
5
4
3
2
1
0
2000
2001
Year
(c)
7
Parameter estimate
6
5
4
3
2
1
0
2000
2001
Year
GS/Winter Feeding II/Appendices CSG15/Appendix 4b
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