Visitors thrive on our hospitality

THE blackcap was so much in the news this winter that it has become a well-known bird.

It used to be thin on the ground in Ireland. Up to the 1970s there were none at all west of the Shannon and, even in eastern counties, the jerky low-pitched warble was not a common sound. I remember vividly the first time I caught one. It had blundered into my mist net in a North Dublin orchard. The celebrity was removed from the net with great care but my inexperienced hands shook with excitement and the bird escaped before I had ringed it. That was over 30 years ago. By then, blackcap numbers were increasing. Soon, the distinctive song could be heard in broad-leaved woods almost everywhere and some blackcaps were spending the winter here.

This elegant soberly-attired bird can be difficult to spot in the mature woods where it nests. About the size of a sparrow, the adult male wears a black skullcap. Females and juveniles have brown ones. The rest of the plumage is pale brownish grey. Everybody assumed the blackcaps which began wintering in Ireland were local breeders or youngsters which had fledged here during the previous season. However, as more birds were ringed, a strange pattern began to emerge. Those ringed in Britain and Ireland in summer were found later in France, Spain and North Africa but not here. But if our wintering birds weren’t locals, where were they from? Then birds ringed in central Europe began turning up. As more rings were recovered, it became evident that many of our blackcaps were German. This was an extraordinary turn of events; Ireland is not exactly a paradise for wintering songbirds. German birds with the fat resources to do so would be better advised to spend the winter in Spain. What could possibly persuade them to head for Britain and Ireland instead of the sunny south?

In 1992, ornithologist Peter Berthold came up with an explanation. He proposed that there were two groups in the German blackcap population; traditionalists which had a propensity to fly south and a newly-formed breakaway group which preferred to head north-west towards Britain and Ireland. Genes determine the general direction in which a bird is disposed to move. Occasionally, however, an individual inherits an urge to experiment or to travel in the wrong direction. One would expect a blackcap heading north-west in the autumn to come to grief. However, birds arriving in Britain and Ireland, Berthold argued, not only survive, they prosper. This part of the world is hospitable to songbirds because people put out food for them in winter. The German visitors thrived during their winter holidays and returned in spring in peak condition. When they bred, their babies benefited from their good fortune; genes promoting north-western migration survived and a sub-population of northwester migrating blackcaps built up.

In normal circumstance, such birds would interbreed with the ones returning from Spain and the tendency to migration westwards would be diluted; genes promoting the traditional movement pattern, being more numerous, should predominate. However, the birds visiting Britain and Ireland had a shorter distance to travel on their migration and didn’t have to fly over the Alps. Therefore, the researchers argued, they arrived back in Germany before the Spanish ones and were less exhausted by their journey. The British and Irish contingent, therefore would start nesting earlier. The two groups, although breeding in the same areas, bred at slightly different times and so north-west orientating genes were transmitted intact to a new sub-population. After a mere 50 generations, a separate race of blackcaps is beginning to evolve.

DNA analysis at the University of Freiburg has confirmed Berthold’s idea that there are two emerging groups. A possible objection to the thesis occurs to me, however. If winter conditions here are so favourable for blackcaps, why don’t British and Irish ones stay on? The German birds incur heavy costs migrating here, although not as great as would be encountered going to Spain.

An Irish blackcap staying put for the winter would avoid the risks and costs of migration completely. So why do Irish blackcaps leave a perfectly viable habitat for foreigners to move in and exploit? Are our birds less streetwise, or more stupid, than German ones?