Eurasian Blackcap - Behaviour

Behaviour

This small passerine bird is partially migratory; central and northern European breeders winter in southern Europe and north Africa, where the local populations are resident. It is hardier than most warblers, partly because it will readily eat small berries as well as the more typical warbler diet of insects.

In recent years, substantial numbers of central European birds have taken to wintering in gardens in Great Britain, Ireland, the Benelux countries, and even southern Scandinavia, migrating northwest or north, instead of southwest. Presumably the ready availability of food, particularly from bird tables, and the avoidance of migration over the Alps and the Sahara Desert compensate for the sub-optimal climate. Bearhop et al. (2005) reported that German birds wintering in England tend to mate only among themselves, and not usually with those wintering in the Mediterranean or western Africa. This is because the short-distance migrants arrive back from the wintering grounds for breeding earlier than birds wintering around the Mediterranean, and form pairs before Mediterranean-wintering birds arrive. The authors point out that division of a population by different migration routes can be a first step towards speciation. The increasing populations have been traced to a tiny population of Eurasian Blackcaps caught in Germany which exhibited a tendency to migrate in a north-westerly direction (instead of the majority that migrate southwards across the Alps to Africa) - the combination of more food and milder temperatures in Britain means that the birds that migrate from Germany to Britain are now apparently at an advantage over those migrating south. It has been postulated that they are spared the long flight to and from Africa and their overwinter survival rate may be relatively high and they may also gain better breeding grounds and territories as they return earlier in the spring than the birds that winter in Africa. This recent change has been related to the recent environmental changes in Britain as the Blackcap populations wintering in Britain did not have this survival advantage and hence the populations were much smaller.