Is supplementary feeding in gardens a driver of evolutionary change in a
migratory bird species?
Kate E. Plummer1*, Gavin M. Siriwardena1, Kate Risely1 and Mike P. Toms1
1
British Trust for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU, UK.
Corresponding author: Kate E. Plummer, Email: kate.plummer@bto.org, Tel: +44 (0)1842
750050, Fax: +44 (0)1842 750030.
Supporting Information
Appendix S1. Annual variation in blackcap occurrence in Britain.
Appendix S2. Complete methods and results of blackcap food use survey.
Appendix S3. Model selection table for GLMM analyses to identify the best predictor of
large-scale spatial variation in blackcap occupancy of GBW sites during
winter.
Appendix S4. Calculation for the change in the amount of supplementary food provisioned at
GBW sites over time.
Appendix S5. Methods and results for estimating the change in winter temperature in Britain
between winter 1950 and 2010.
Appendix S6. Evidence of British breeding blackcaps remaining in Britain during winter
using ringing recoveries and recapture data.
1
Appendix S1
Annual variation in blackcap occurrence in Britain
Variation in blackcap (Sylvia atricapilla) reporting rates across the annual cycle was
evaluated to determine the appropriate winter timeframe for analysis, in order to avoid the
inclusion of summer migrants.
Weekly records of blackcap presence/absence from the British Trust for Ornithology (BTO)
Garden BirdWatch (GBW) were averaged for years 1999 to 2011, using data from 3,806 sites
throughout Britain selected for analysis (n = 2,201,509). On average wintering blackcaps
arrived into GBW gardens from early November, with occurrence peaking between lateJanuary and early March and then declining again through late March and April (Fig. S1a).
Blackcaps were recorded much less frequently in gardens between April and October.
However, data from the BirdTrack scheme (www.birdtrack.net, established in 2005), where
volunteers submit complete lists of all species seen and heard for a given birdwatching event,
can be used to assess annual variation in the presence of blackcaps within Britain beyond
only gardens. The proportions of complete lists submitted each week containing blackcap
were averaged for years 2005 to 2011 (n = 433,720). On average the earliest summer
migrants arrived in late March, with occurrence increasing rapidly through April and peaking
at the beginning of May then declining in the autumn (Fig. S1b).
Given this evidence, winter was defined as a 21-week period encompassing months
November to March. This captures the full extent of the usage of gardens by blackcaps in
winter during the years under investigation, but limits the possibility of including summer
migrants which largely arrive in Britain during April and utilise the wider countryside.
2
Figure S1. Weekly reporting rates of blackcaps in Britain using two contrasting data sources.
(a) The smoothed annual trend in the proportion of GBW gardens in Britain (n = 3806) where
blackcap(s) occurred in 1999 – 2011 (± 95% CI). (b) The smoothed annual trend in the
proportion of BirdTrack complete lists per week where blackcap(s) occurred in 2005 – 2011
(± 95% CI). Dotted lines represent the beginning and end of the winter period identified for
analysis.
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Appendix S2
Identification of key supplementary feeding resources for blackcaps
As part of the GBW weekly recording form, surveyors submit presence-absence data for the
types of supplementary foods they have provisioned each week. Supplementary food is
categorised into 11 all-encompassing food types (see Fig. S2). We conducted an independent
survey to identify key artificial feeding resources utilised by blackcaps. The survey was
targeted at GBW participants (n = 900) who observed blackcaps in the most recent winter
(2013/14) and provided at least six food types during the weeks in which blackcaps were
recorded. Responses were collected using an online survey tool (SurveyMonkey,
www.surveymonkey.com) from 2 May – 2 June 2014. Respondents were asked to identify
which food types had been provisioned during November 2013 – March 2014, and whether
or not blackcaps had been seen feeding on them. From 534 responses, blackcaps were most
often seen feeding on fats (64.3% of sites where food type was fed) or sunflower hearts
(50.4%) (Fig. S2). As such these foods were defined as key food resources for blackcaps,
and their influence on blackcap occupancy was further investigated.
Figure S2. Blackcap supplementary food preferences. Numbers correspond to the total
number of respondents provisioning each food type, and percentages represent the
proportions of those which observed blackcaps using the food. All food types recorded in
Garden BirdWatch are reported.
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Appendix S3
Table S3 | Model selection table for GLMM analyses to identify the best predictor of largescale spatial variation in blackcap occupancy of GBW sites during winter. Spatial predictors
tested comprised REGION, latitude (LAT), longitude (LONG) and average regional temperature
(TEMP). Nonlinear relationships for continuous predictors were also tested by fitting models
that included quadratic terms.
AIC
k
AIC
∆AIC
weight
R2GLMM(m)
R2GLMM(c)
12
20706
0.0
> 0.99
0.111
0.465
6
20739
33.2
0.00
0.105
0.467
LAT + LONG
4
20815
109.3
0.00
0.081
0.464
LAT + LONG + LAT:LONG
5
20817
111.1
0.00
0.081
0.464
4
21007
301.2
0.00
0.045
0.475
LONG + LONG2
4
21021
315.3
0.00
0.045
0.474
2
TEMP + TEMP
4
21030
324.8
0.00
0.039
0.474
LAT
3
21054
348.2
0.00
0.030
0.474
TEMP
3
21058
352.1
0.00
0.031
0.475
LONG
3
21071
365.7
0.00
0.028
0.473
null
2
21184
478.6
0.00
0.000
0.478
Fixed variables
REGION
2
LAT + LONG + LAT
LAT + LAT
2
2
+ LONG
5
Appendix S4
Estimating changes in bird feeding practices among GBW participants
Assumptions regarding weekly amounts of bird food provisioning by GBW participants:

A commercially bought fat ball is estimated to weigh 100g

An average seed feeder has an estimated capacity of 500g

As a conservative estimate of bird feeding habits and food consumption rates, an average
homeowner is assumed to provision two fat balls per week and/or 250g of seed per week.
Table S4 | Estimated change in total amounts of sunflower hearts and fat products
provisioned during winter at analysed BTO Garden BirdWatch sites from 1999 to 2010.
Average number of weeks foods
were provisioned
Average mass of food
provisioned per garden (kg)
# GBW
sites
Sunflower
Fats
Sunflower
+/or Fats
Sunflower
Fats
Total food
mass in GBW
sites (tonnes)
1999
2407
4.08
8.09
10.08
1.02
1.62
6.35
2000
2916
5.00
9.12
11.35
1.25
1.82
8.96
2001
3273
5.92
9.54
12.19
1.48
1.91
11.09
2002
3644
6.75
10.27
13.12
1.69
2.05
13.63
2003
3659
7.35
10.98
13.91
1.84
2.20
14.76
2004
3691
7.95
11.40
14.40
1.99
2.28
15.75
2005
3663
8.79
12.14
15.14
2.20
2.43
16.94
2006
3604
9.20
11.39
14.83
2.30
2.28
16.50
2007
3646
9.58
12.20
15.44
2.40
2.44
17.63
2008
3534
9.70
13.05
15.83
2.43
2.61
17.79
2009
3377
9.86
13.71
16.28
2.47
2.74
17.58
2010
3126
10.41
14.60
16.85
2.60
2.92
17.26
Winter
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Appendix S5
Estimating the change in winter temperature in Britain between 1950 and 2010
Annual mean winter temperature for Britain was calculated using the average of TEMP (see
Table 1) for all 5-km squares in Great Britain in each winter. Change in winter temperature
over the period in which the British wintering blackcap population has become established
(since the 1950s) was quantified using a linear regression of national winter temperature
against winter year. British Met Office UK Climate Projections (UKCP09) gridded data is
available up to the winter of 2010.
In the 60 years over which the blackcap wintering population has become established in
Great Britain, mean winter temperature has increased steadily from an estimated 3.70oC in
1950 to 4.90oC in 2010 (0.02oC yr -1, F1, 59 = 12.18, p < 0.001; Fig. S5).
Figure S5. The trend in British winter temperature from 1950 to 2010. The line shows
predicted values from the linear model.
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Appendix S6
Evidence of British breeding blackcaps remaining in Britain during winter
Blackcaps breeding in Britain predominantly migrate to southern Spain and northwest Africa
for the winter months, with some perhaps crossing the Sahara (Mason, 1995, Wernham et al.,
2002). Prior to 1998, only a single blackcap confirmed as a British breeder was reported to
have remained in Britain during winter, while there was growing evidence that increasing
numbers of wintering birds reflected a change in the migratory behaviour of the central
European population (Wernham et al., 2002). It is still possible, however, that increased
sedentary behaviour among British breeders has occurred. Using the same definitions of the
summer and winter seasons as those used for The Migration Atlas (Wernham et al., 2002), we
have evaluated recent evidence of sedentary behaviour (i.e. wintering in Britain) of breeding
blackcaps using recoveries of dead birds ringed under the British Trust for Ornithology
(BTO) Ringing Scheme (Table S6). Summer is defined as 1 June – 15 August and winter is
defined as 1 December – 14 February. As such, the data are restricted to periods where
movements are at a minimum (Wernham et al., 2002). Furthermore, we have examined
evidence of individual blackcaps being ringed and recaptured in Britain in summer and
winter, or vice versa (Table S6). To investigate changes in the distance of British breeders
are migrating, we used linear regression to test the trend in migration distance over time using
all recoveries of dead birds that had been ringed in summer and recovered in winter, or vice
versa, since 1950 (n = 76).
Annual totals of blackcaps ringed in Britain increased, although not always smoothly, both
before and after 1998 (Robinson & Clark, 2014). Therefore, this increased ringing effort is
likely to result in a greater number of summer-ringed birds being found in Britain during
winter over time, by chance. However, there is no evidence that this has occurred to date,
with just one further winter recovery since the previous analysis by Wernham et al. (2002)
was completed (Table S6). Recaptures of live birds between summer and winter nominally
provide an additional source of evidence of movements, although also influenced by variation
in catching effort. There have been four such recaptures, all since 2001 (Table S6). Note,
however, that all these recaptures were made at the site of ringing and that such zero-distance
movements were not recorded in the ringing database before 2000. Hence, it is unknown
whether a similar rate of apparent sedentary behaviour also occurred in earlier years. British
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blackcaps have mostly been recorded wintering in Morocco, Spain, Algeria and Portugal.
There is no evidence that the distance British breeders are migrating has changed over time
(F1,74 = 0.13, p = 0.718). It is likely that the probability of ringing recoveries being reported
is considerably lower in southern Europe and northern Africa than it is in Britain, but there is
no reason to suspect that this spatial bias has changed over time, so a reduction in migratory
distance ought still to be apparent if it had occurred.
Overall, it remains possible that British breeding blackcaps are becoming more sedentary, but
there is no evidence that any such change has involved large numbers of individuals, or a
large proportion of the winter population, to date. Furthermore, there is no clear evidence
that British breeding blackcaps have shortened their migration distance over time, which
might have suggested the process leading to sedentary behaviour was underway (Pulido,
2007). Winter records of otherwise migratory species are a regular occurrence, so while they
may show some flexibility in behaviour, they do not necessarily indicate developing changes
at the population level.
Table S6 | Evidence of British breeding blackcaps remaining in Britain during winter using
ringing recoveries and recapture data. Data are presented for individuals ringed in Britain
during either the core breeding or wintering periods (see text for definitions) and later
recovered or recaptured in a different season (winter or summer respectively). Note that all
recaptures reported were at the same sites at which the birds involved had been ringed
originally.
Ringing
Date
Recovery
Age
Sex
Region
Date
Region
Ringed in summer and recovered in winter
* 02 Aug 1990
09 Jun 1997
Juvenile
F
East Midlands
11 Feb 1996
South East
Fully-grown
M
East of England
27 Dec 1997
South East
Recaptured in a different season
19 Jun 1999
Juvenile
M
East of England
29 Jan 2001
East of England
23 Jun 2001
Juvenile
M
South West
26 Dec 2001
South West
09 Aug 2011
Juvenile
-
South West
30 Jan 2012
South West
30 Jan 2012
Fully-grown
F
South East
26 Jul 2012
South East
* Previously reported in The Migration Atlas(Wernham et al., 2002).
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References
Mason CF (1995) The Blackcap, London, Hamlyn.
Pulido F (2007) The genetics and evolution of avian migration. BioScience, 57, 165-174.
Robinson RA, Clark JA (2014) The Online Ringing Report: Bird ringing in Britain & Ireland
in 2013 (http://www.bto.org/ringing-report, created on 30-May-2014). Thetford,
BTO.
Wernham CV, Toms MP, Marchant JH, Clark JA, Siriwardena GM, Baillie SR (2002) The
Migration Atlas: Movements of the Birds of Britain and Ireland, London, T. & A.D.
Poyser.
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