With the public's help, we wanted to find out more about the behaviour of Blackcaps in winter gardens, focusing on three key questions:

• Which foods are Blackcaps eating? Garden bird food seems to be a key ingredient for the survival of UK-wintering Blackcaps. However, we didn't have any data which explored their feeding habits in detail. 
• Are there equal numbers of male and female Blackcaps? We were interested to know if there was a sex bias in the use of gardens by Blackcaps in winter, since this could have pointed towards different migratory patterns of males and females.
• Are Blackcaps aggressive with other similar-sized birds? Blackcaps have a growing reputation for being grumpy at bird feeders, shooing off other birds that come to feed. But is this reputation justified? We wanted to explore interactions between Blackcaps and other, similar-sized birds at bird feeders.

What is enabling the growth in our wintering Blackcap population?

We also used the weekly observations submitted by BTO Garden BirdWatchers to look at the suite of factors (including climate change and food provision) that might be behind the increase seen in gardens during the winter months.

We used Blackcap presence/absence data from Garden BirdWatch sites for 12 winters (1999/2000 to 2010/11), capturing the period when wintering Blackcaps are most strongly associated with garden habitats. The final data set included 3,806 Garden BirdWatch sites. While we had data on which sites provided different foods, we didn't know which foods the Blackcaps were favouring, hence the addition of a food preference question in our study.

The analyses also included a measure of local habitat, derived from the CEH Land Cover Map and used to test for any potential ‘heat island effect’ – urban areas are warmer than rural areas because of the waste heat escaping from buildings and shops. Also included were mean monthly temperature data extracted from the Met Office UK Climate Projections dataset, latitude/longitude and year. Statistical models were then used to examine the predictors of variation in Blackcap wintering behaviour.

From the responses to the questionnaire, we found that fat-based foods and sunflower seeds were the two food types that Blackcaps preferred at garden feeding stations. When it came to the numbers of males and females using gardens, we found that the majority of participants saw a maximum of one bird in their garden, whether it was male or female. Blue Tits seemed to the birds that were scared off most easily by Blackcaps, and Robin the least likely, but many people saw no aggression at all.

Analysis of the Garden BirdWatch data revealed that Blackcaps showed greater occupancy of sites in the south and west of Britain, where wintering conditions are milder. We also found strong evidence that the use of garden sites was influenced by both supplementary food and climatic temperature; Blackcaps were recorded more often at sites that provisioned food more frequently and, most interestingly, Blackcap occurrence has become more strongly associated with supplementary feeding over time. The birds showed a preference for wintering sites that had a warmer local climate, with the use of sites reduced in those years when the winter weather was milder.

This work provides the first direct evidence of the underlying mechanisms that have influenced the contemporary evolution of migratory behaviour in Blackcap. Over a 12-year period, Blackcaps have become increasingly associated with the provision of supplementary foods in British gardens and the reliability of that provisioning is influencing their distribution at the national scale. The findings suggest that climate amelioration is also likely to have enabled Blackcaps to expand their wintering range into Britain.

The increasing association with supplementary food over time suggests that Blackcaps are adapting their feeding habits to exploit human-provisioned foods, complementing recent evidence that those Blackcaps migrating to Britain in winter are diverging phenotypically, as well as genetically, from those that winter in Spain. Blackcaps wintering in Britain have relatively narrower and longer beaks than those wintering in Spain, suggesting that British migrants have adapted to a more generalist diet.

The study provides new and timely evidence of the role that human activities can play in shaping the evolutionary trajectories of wild bird populations.

The results of this work were published in a peer-reviewed paper. Garden BirdWatch participants have also contributed to other BTO work on wintering Blackcaps.