Backyard bird feeding sparks a songbird ‘reverse migration’

ITHACA, N.Y. – Eurasian Blackcaps are spunky and widespread warblers that breed across much of Europe. Many of them migrate south to the Mediterranean region and Africa after the breeding season. But thanks to a changing climate and an abundance of food resources offered by people across the United Kingdom and Ireland, some populations of Blackcaps have recently been heading north for the winter, spending the colder months in backyard gardens of the British Isles.

New research published this week in Global Change Biology shows some of the ways that bird feeders, fruit-bearing plants, and a warming world are changing both the movements and the physiology of the Blackcaps that spend the winter in Great Britain and Ireland.

“Many migratory birds are in decline, but the Blackcap seems to be thriving in a changing world,” says Benjamin Van Doren, a postdoctoral researcher at the Cornell Lab of Ornithology and lead author of the paper.

The research team, consisting of scientists from Oxford University, British Trust for Ornithology, and Max Planck Institute, banded hundreds of birds at dozens of sites across the British Isles and recruited community scientists to report sightings of the birds at feeders over the course of four winters. They also tracked several dozen individuals with geolocators to follow their migrations throughout the year.

Their study showed that Blackcaps wintering in the British Isles move around much less than their southern-wintering counterparts–especially later in the winter. They are also more likely to return to the same sites from one winter to the next. The authors believe the availability of food is the main reason for the difference.

“Birds that winter in the Mediterranean and Africa are primarily eating fruit and moving large distances to track local food abundance,” says Van Doren. “Blackcaps wintering in British and Irish gardens have a steady, predictable food supply, and as a result they move around a lot less.”

Additionally, the British- and Irish-wintering Blackcaps appear to be in better shape. Researchers found birds frequenting gardens to be in better body condition and maintain smaller fat reserves than the more transient individuals in the study–and birds carrying less fat have an easier time avoiding predators. These birds were also able to make a speedier return journey in the spring, arriving back around 10 days earlier than those wintering in the Mediterranean and Africa–a considerable advantage at a crucial time of year.

The researchers even found that gardens may be influencing Blackcap anatomy: those in gardens had longer bills and more rounded wingtips, traits that may be linked to their more generalist diet and sedentary winter lifestyle.

“Our results show that individual Blackcaps have great flexibility in their movement patterns and how they respond to environmental conditions,” Van Doren says. “Species with this kind of flexibility will probably be better equipped to face environmental changes in the coming decades.”

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Reference: Benjamin Van Doren, et al. Human activity shapes the wintering ecology of a migrating bird. Global Change Biology. April, 2021. https://onlinelibrary.wiley.com/doi/10.1111/gcb.15597