Why blackcaps altered their migration pattern

EXPLOITATION and global warming are changing habitats worldwide. Many plants and animals are in trouble.

EXPLOITATION and global warming are changing habitats worldwide. Many plants and animals are in trouble, writes Richard Collins.

Birds, being able to fly, cope better than most. Fulmars, for example, began breeding on Irish sea-cliffs in 1911. A new food source, offal from trawlers and whaling ships, had tempted them to venture south of sub-Arctic waters. Few Bewick’s swans from Siberia spend the winter in Ireland nowadays; with a warming climate, they no longer need to come this far.

Over the last few decades, blackcaps have altered a migration pattern that had taken thousands of years to perfect. This grey-plumaged, sparrow-sized bird isn’t easy to spot; it stays under cover, but will visit gardens for food hand-outs. Males have black skull-caps. The females and juveniles are brown. Traditionally regarded as summer migrants, blackcaps were rarely seen in winter. “The mildness of the Irish climate may be connected with these cases,” Richard Ussher and Robert Warren remarked, prophetically, in 1900. Nowadays, blackcaps are here year-round.

The wintering individuals, it was assumed, were local breeders, opting to remain on rather than migrate. However, as more birds were ringed during the winter, it became clear that this was not the case. Our blackcaps were turning up in mainland Europe, rather than Ireland, in the breeding season. So, if our winter ones weren’t locals, where were they from? As ringing continued, an answer began to emerge; our wintering blackcaps were from southern Germany.

German blackcaps normally head south, towards Spain, in the autumn. Why were they travelling north-westwards to dark, damp Britain and Ireland, instead? The general direction of bird migration is genetically determined, but, occasionally, individuals choose the ‘wrong’ direction. For a German blackcap to do so would normally be fatal. The birds wintering in Britain and Ireland, however, survive and prosper. Food provided in our gardens and parks helps them through ‘the bleak midwinter’, in conditions rendered relatively mild by climate change.

Our blackcaps, seemingly, return to Germany fit and well. Their southward-migrating peers, on the other hand, have to get past that formidable barrier, the Alps. Having travelled farther, they arrive home weaker, perhaps finding the best territories already taken by their British and Irish cousins. At a disadvantage when breeding, their genes lost out to those of the north-westerly migrants and the alternative winter destination became established.

But is this the whole story? Benjamin Van Doren and Greg Conway, writing in the current edition of Lifecycle, ask some interesting questions. Do some of our wintering blackcaps come from other parts of Europe? If winter conditions here are so favourable, why don’t visiting ones stay on to breed, avoiding the trials and tribulations of flying home? Among migrant species generally, individuals travelling farther tend to have longer wings. Are the wings of the blackcaps that go to Spain longer than the wings of those who visit us? Blackcaps might be doing well, but how are other species faring? Are today’s environmental changes occurring too quickly for birds to keep up’?

To answer such questions, researchers are deploying state-of-the-art technology. Thirty-six blackcaps were fitted with geo-locators last winter. By logging ambient light levels, these devices enable a bird’s movements to be tracked. Meanwhile, blackcaps are being colour-ringed. This will provide additional information and help ringers find and re-trap geo-locator-carrying birds, so that the data in their monitors can be retrieved. ‘A combination of approaches will probably be necessary to reveal how migration is being modified in a changing world’, the scientists say.

Benjamin Van Doren & Greg Conway, ‘Blackcap migration mysteries,’ Lifecycle, 2017.

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