The British Trust for Ornithology has been collecting data about the migratory behaviour of blackcaps.
Since the 1960s more and more blackcaps are spending the winter here. Unlike our chiffchaffs, these are not the same birds that come here in the summer to breed. They are two distinct populations.
The summer blackcaps that come to the UK to breed, leave in the autumn and migrate south, most usually to the Iberian Peninsula and North Africa. The blackcaps that come here in the winter are a separate population that has reared its young in central Europe, mostly in southern Germany and Austria, and migrated here, to southern Britain, at the end of the summer.
Over wintering blackcaps’ behaviour also differs from that of the summer population: winter blackcaps have become garden based birds, dependent on urban bird-feeders for a diet of fats and sunflower seeds while summer blackcaps are insectivores based in the countryside.
 female blackcap (with a brown cap) at a bird feeder  male blackcap singing
Audio: Eurasian Blackcap (Sylvia atricapilla) from xeno-canto by Jerome Fischer.
The BTO’s data shows that it’s not just the feeding behaviour of the two populations that is changing, their appearance is changing too. Blackcaps wintering in Britain have relatively narrower and longer beaks than those wintering in Spain, suggesting that the British migrants are adapting to their more generalist diet.
The UK’s warming climate, which may be what originally attracted migrating blackcaps, and the British bird-feeding habit, which has enabled them to succeed here, are driving these adaptation. Scientists believe that the two populations are becoming genetically separate because birds that overwinter in Britain are more likely to choose partners that do the same. Some believe that the two populations are actually in the process of becoming two different species.
Keep watch for our breeding blackcaps this summer in the park and, come winter, at you garden’s bird feeder.