Human activity shapes the wintering ecology of a migratory bird

Male Blackcap. Benjamin Van Doren

Author(s): Van Doren, B.M., Conway, G.J., Phillips, R.J., Evans, G.C., Roberts, G.C.M., Liedvogel, M. & Sheldon, B.C.

Published: April 2021   Pages: 13pp

Journal: Global Change Biology

Digital Identifier No. (DOI): 10.1111/gcb.15597

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New research involving BTO provides further evidence of how humans are shaping the natural world through simple changes in our own back gardens.

Blackcaps typically visit Britain and Ireland during the spring and summer months to breed before migrating south to wintering areas in the Mediterranean. However, in recent decades, Blackcaps have expanded their wintering range northward across Europe and are now frequently found in Britain and Ireland during the winter months. These birds are not breeders staying put, but originate from breeding locations spanning a 2,000 km stretch of Europe, with birds undertaking a highly atypical north-westward migration each autumn.

This study, in collaboration with Oxford University and the Max Planck Institute, used data from colour-ringing to investigate the movements and behaviour of Blackcaps wintering in Britain and Ireland. A sample of 32 birds was also tracked with geolocators, which measure day length and the time of solar noon, and give more detailed information on birds’ movements than colour-ring sightings alone. The birds were ringed and their movements monitored by volunteers taking part in the Ringing Scheme and Garden BirdWatch.

The results showed the extent to which Blackcap ecology is shaped by human activities. The provision of bird food in gardens during cold weather has not only contributed to evolutionary change in Blackcaps, in that large numbers now migrate to Britain and Ireland for the winter, but has also modified the behaviour of the birds once they are on their wintering grounds. Blackcaps wintering in the Mediterranean primarily eat fruit and move through the landscape once their existing food supply has been depleted. In contrast, Blackcaps wintering in British and Irish gardens have a steady, predictable food supply. As a result, they move around less and are more faithful to particular wintering sites.

The study also found stark differences in the birds’ anatomy and physiology depending on how often they appeared in gardens. For example, Blackcaps that frequent gardens and therefore have a predicatable food supply, are able to carry smaller fat stores, reducing their weight and making them more agile and likely better able to escape from predators. However, Britain and Irish-wintering Blackcaps are able to fatten up fast before migrating back to their breeding grounds and make a speedy return journey (arriving back around 10 days earlier than their competitors), which is another possible advantage of having a predictable food supply. The research showed that gardens might even be influencing Blackcap anatomy: birds in gardens have longer bills and more rounded wingtips, which may be linked to their more generalist diet and sedentary lifestyle.

This study is a fantastic example of cooperation, where BTO volunteer Garden Birdwatchers and bird ringers provide the data that underpins robust scientific research into understanding the influence of human activity on garden birds. At a time when so many species are struggling to adapt to human-mediated environmental change, Blackcap numbers are actually rising and this species’ ability to ring in the changes might explain why.


Human behavior profoundly affects the natural world. Migratory birds are particularly susceptible to adverse effects of human activities because the global networks of ecosystems on which birds rely are undergoing rapid change. In spite of these challenges, the Blackcap (Sylvia atricapilla) is a thriving migratory species. Its recent establishment of high-latitude wintering areas in Britain and Ireland has been linked to climate change and backyard bird feeding, exemplifying the interaction between human activity and migrant ecology. To understand how anthropogenic influences shape avian movements and ecology, we marked 623 wintering Blackcaps at 59 sites across Britain and Ireland and compiled a dataset of 9,929 encounters. We investigated visitation behavior at garden feeding sites, inter-annual site fidelity, and movements within and across seasons. We analyzed migration tracks from 25 geolocators fitted to a subset of individuals to understand how garden behavior may impact subsequent migration and breeding. We found that Blackcaps wintering in Britain and Ireland showed high site fidelity and low transience among wintering sites, in contrast to the itinerant movements characteristic of Blackcaps wintering in their traditional winter range. First-winter birds showed lower site fidelity and a greater likelihood of transience than adults. Adults that frequented gardens had better body condition, smaller fat stores, longer bills, and rounder wingtips. However, Blackcaps did not exclusively feed in gardens; visits were linked to harsher weather. Individuals generally stayed at garden sites until immediately before spring departure. Our results suggest that supplementary feeding is modifying Blackcap winter ecology and driving morphological evolution. Supplemental feeding may have multifaceted benefits on winter survival, and these positive effects may carry over to migration and subsequent breeding. Overall, the high individual variability in blackcap movement and foraging ecology, and the flexibility it imparts, may have allowed this species to flourish during rapid environmental change.

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The authors thank Sally Amos, Jon Avon, Jake Bailey, Penny Barret, Stuart Bearhop, Rob and Liz Boon, Stuart Brown, Malcolm Burgess, Emily Cuff, Kate Dalziel, Kira Delmore, Davide Dominoni, Ian Duncan, Rachel Durham, Phil Evans, Sheila Evans, Andrew Farnsworth, Kate Fox, Roger Francis, Lyn Gammage, Gill Garrett, Sheila Gowers and Paul Ensom, Mark Grantham, Jodie Mae Henderson, John and Jane Holmes, Emma Inzani, Brian Isles, Michael and Helen Johnson, Ali Johnston, Javier Pérez-Tris, Ellie R. Ness, Mel Mason, Irene McGregor, Keith McMahon, Eliot Miller, Nicole Milligan, Ruedi Nager, Ben Porter, Dee and Jonnie Reeves, Fiona Roberts, Dr ET Roberts, Gary Samways, Ash Sendell-Price, Ana Shapiro, Anna Smith, Dave Stoddard, Erica Stuber, Esmé Tackley, John Webber, Kester Wilson, Penny Witcombe, Connor Wood, and other contributors, ringers, and homeowners.

The BTO Ringing Scheme is funded by a partnership of the British Trust for Ornithology, the Joint Nature Conservation Committee (on behalf of Natural England, Natural Resources Wales and NatureScot and the Department of the Environment Northern Ireland), The National Parks and Wildlife Service (Ireland) and the ringers themselves.

This work was supported through funding from the Max Planck Society, the Marshall Aid Commemoration Commission, the American Ornithological Society, the Society for the Study of Evolution), the Frank M. Chapman Memorial Fund, and the British Trust for Ornithology.

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