Diversity of migration in European blackcaps (Sylvia atricapilla)

Project Description

Bird migration is changing, in response to changes in climate and habitat. Some species are declining because their breeding, wintering or stopover sites have been removed for development. Others have expanded their wintering range, or become partial migrants, because our climate has warmed enough that they no longer need to migrate away from temperate areas in autumn.
Most notably, in the past 50 years the European blackcap (Sylvia atricapilla; from here-on, Blackcap), a species that typically overwinters in the southern Mediterranean or north Africa, has started to regularly overwinter in the United Kingdom. In fact, these birds are not just UK breeders, but also birds that have bred in Central Europe, that have rapidly evolved a new migratory route in a north-westerly direction. This divergence from a typical south-easterly or south-westerly route has been picked up by ringing recoveries and, more recently, geolocators.
As a result of these changes, migratory populations may experience changing survival rates, and new phenotypes may become established through changes in morphology and physiology. If increased survival allows genes passed on through assortative mating, this divergent migration may contribute towards speciation. However, understanding the causes and consequences of changes in migration is challenging, because of the vast areas covered by many migratory journeys, and the lack of appropriate technology – particularly for small passerines.
For the blackcap, small sample sizes and a lack of detail in the data collected by miniature geolocators mean that we still lack information on migratory connectivity and routes used. Multiple novel routes to and from the UK might be used by continental birds, which may also converge with blackcaps of UK breeding origin and may be variable, or consistent, between individuals, leading to identification of population-specific routes that could influence demography.
This project will use recently developed miniature VHF tags, deployed onto migrating blackcaps in the UK at several key locations on the east coast. These are sub 1g tags, which emit a uniquely-coded signal, able to be detected by automated stationary VHF receivers. Tags will be deployed at one of the six receivers on the east coast of the UK, using a temporary attachment that will last long enough to detect departure directions from the deployment site. Data collected on departure directions highlight the direction of travel of the individual, and will indicate whether birds are incoming migrants to the UK to breed, or outgoing migrants to the continent, having wintered in the UK.
These will be collected alongside feather samples to analyse stable isotopic signatures to identify the individual’s moult location.
These data will highlight the variety of migratory routes converging in the UK and will increase our understanding of the evolution of new routes within a species, helping us to understand more about species’ adaptation to anthropogenic change.