Abstract

The migration routes and wintering areas of Blackcaps Sylvia atricapilla are changing. Conventional geotagging techniques gave broad-scale data on movements but required the bird to be recaught. New nanotags, which run using the Motus network, have allowed for fine-scale tracking of small birds, including indicating the direction of departure away from Motus base-station receivers. Deploying Motus tags on Blackcaps has already yielded interesting data regarding the direction of departure of birds trapped during spring and autumn at sites in southern and eastern England.IntroductionThat bird migration routes and breeding and wintering locations are changing is well known (see, e.g., Zurrell et al. 2018). However, the complexity of the interactions between the life histories and ecology of the birds as well as environmental and anthropogenic changes makes these changes hard to predict. While many bird species are declining due to climate change, which is pushing them to the extreme of their habitable ranges (Cook et al. 2021), there are some species that are able to expand their range or shift their migratory route to compensate for changing conditions (Rushing et al. 2020). This can happen when, for example, adaptation in the form of new phenotypes is able to take place within a few generations, an evolutionary change termed microevolution. The Blackcap Sylvia atricapilla is a notable example of a species that is showing rapid microevolution; in this case, birds from populations in northern Europe that previously wintered in Africa or the southern Mediterranean have begun wintering – via a reverse migratory route – in Britain (Berthold 1995). Research by Hiemer et al. (2018) and van Doren et al. (2021), which used geolocators to track birds, has shown that the individuals wintering in Britain originate from a widespread breeding area across the European continent, as far east as Poland and the Czech Republic (Delmore et al. 2020). The change in migration route and ability to overwinter in Britain is likely due to a combination of warming winter conditions in Britain and a proliferation of feeders in gardens (Plummer et al. 2015), which both facilitate overwinter survival. Migratory direction and timing in Blackcaps are heritable (Berthold 1995; Pulido et al. 2001) and, because of the repeated return of birds to Britain in winter, we know there must be strong fitness benefits for these shorter-distance migrants that have allowed them to continue to overwinter in Britain and, indeed, to prosper (de Zoeten & Pulido 2020). In fact, the species’ wintering distribution in Britain has expanded by more than 50% since the 1981–84 wintering bird atlas (Lack 1986).