One surprising visitor to favoured winter bird tables is the blackcap, a species of warbler more commonly regarded as a summer visitor to these shores. The presence of blackcaps in Britain during the winter is not a new phenomenon but the numbers of individuals wintering here is.
It appears that those blackcaps choosing to overwinter here are not simply birds that bred here during the summer and which have chosen to remain. Instead, they are birds from the central European breeding population, arriving in late autumn and then remaining through until early spring. Some birds from the breeding population have probably always overwintered here but the rapid increase in the numbers that now do this, instead of migrating south to the Mediterranean, is thought to be linked to our warming climate and the number of households providing food, in the form of suet-based products and other bird food.
Research has demonstrated that those blackcaps from the central European breeding population that choose to winter in the UK get back to their breeding grounds ahead of those that winter further south. They then tend to pair with similar individuals and get good territories and show good levels of breeding success. The tendency to travel to the UK for the winter is, therefore, being passed from one generation to the next and we are seeing evolution in action, as a new pattern of autumn migration develops.
One of the most interesting aspects of the winter distribution of these birds within the UK is the way in which they favour the southwest of the country and seem to prefer urbanised landscapes over rural ones. This suggests that winter temperature may be important, as indicated by the birds choosing to winter in the mildest regions and habitats. Temperatures in urban centres may be several degrees warmer than the temperature in the surrounding countryside, a consequence of all of the waste heat lost from our homes, shops and offices.
Another interesting aspect is the behaviour of the blackcaps. It appears that they are somewhat quarrelsome birds and that they will attempt to defend a food resource – such as your bird table – against all-comers. Those birdwatchers whose gardens attract these wintering blackcaps often comment on the way in which they see off robins, finches and other birds. Some gardens may attract several blackcaps over the winter, although many hold just single birds.
It is not clear what, if anything, will happen to those blackcaps that breed here. Will they follow the example of their continental cousins and alter their migratory habits? Might we see them stay here and become residents rather than summer visitors? Who knows, but it will be fascinating to see how things change.