Waterford Wildlife

Wednesday, 11th January 2012

Wintering blackcaps - Blackcap

Forty years ago, the Blackcap was a scarce summer visitor from Africa. Now it is a common winter feeder at Waterford garden bird tables. What happened? How can we explain the sheer numbers of Blackcaps that now choose to spend the winter in Ireland and Western Europe, rather than in their usual wintering areas in southern Europe and North Africa? Why do they do this?

Well, a clue to the first question came as early as December 1961 when a Dublin cat caught a Blackcap that had been ringed in Austria the previous summer. Despite this and other ring recoveries, the breeding origins of wintering birds in Ireland and western Europe was still disputed; some people believed they were breeders that had failed to migrate while others thought they might be of Scandinavian origin.

In the early 1990’s, a research group in Germany finally laid the problem to rest. Professor Peter Berthold and his colleagues captured 40 Blackcaps at Somerset in Western England during the winters of 1988-1990. He took these birds to his research station in southern Germany and held them until the following autumn when the birds begin to show migratory restlessness (they hop in the direction they want to migrate). He found that, in contrast to most of the Blackcaps breeding in southern Germany (which migrate in a south-westerly direction toward Spain and the short hop over sea to North Africa). The British birds orientated in the direction of the UK (and Ireland) suggesting that the breeding origins of the western European population wintering here were most likely in south central Europe.

So, why is it only now that we are seeing a population increase? Well, for a start, winters are much milder now than 50 or 60 years ago, potentially making it easier for these southern birds to survive. Secondly, we have been creating a winter feast for Blackcaps by planting bushes and shrubs that bear berries in our parks and gardens and keeping our bird-tables well stocked. More importantly, it also seems that by wintering here our Blackcaps gain an advantage over the birds that winter in southern Europe and Africa. The reason forit is that the main thing that triggers the urge to migrate is day length. In Ireland, the critical day length is reached around 14 days earlier than it is in the more southerly wintering areas. Therefore, Irish Blackcaps probably arrive at their breeding sites earlier, claim the best territories and start breeding before the Africans arrive.

In addition, because the distance between Ireland and Britain and central Europe is much shorter than that between southern Iberia, north Africa and central Europe, it is likely that our Blackcaps make energy savings and so have more energy to devote to breeding, thereby further increasing their chances of success. They will also tend to rear more young than birds wintering elsewhere. Because the migratory trait is inherited, their offspring will also then winter here and survive better, produce more young and so on: until we reach the point today when Blackcaps have become a relatively common, but always welcome sight at our winter bird-tables. Finally, about 1% of birds ringed is ever recovered. In other words - one in every hundred. Just over 100 have been ringed have now been ringed in Waterford. None have yet been recovered –yet!