Migratory behaviour and population growth of Blackcaps wintering in Britain and Ireland: Some hypotheses

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Abstract
Over the past several decades the number of Blackcaps (Sylvia atricapilla) wintering in Britain and Ireland has increased from a few birds to a substantial wintering population involving thousands of individuals. Ringing recoveries indicate that these birds breed in central Europe and migrate to the British Isles to overwinter rather than going to conventional wintering areas in southern Europe and northern Africa. We discuss possible explanations for the origin of this apparently novel migratory trait. We also propose several hypotheses to account for the extremely rapid increase in this group relative to the population at large. Basically, these hypotheses propose higher survivorship (due to feeding stations) and greater reproductive success of birds that migrate to the British Isles to overwinter relative to birds that migrate to more traditional wintering areas in the Mediterranean region. We hypothesize that a key factor lies in the photoperiod experienced by the British wintering birds which might result in an earlier break in the refractory period and early breeding and/or spring migratory conditions relative to birds wintering farther south. It is suggested that a new ringing programme, another winter census and a series of observations of captive Blackcaps may enable us to refute or accept these hypotheses.