Tuesday, October 25, 2005

Go West, Young Blackcap!

Considering that I will soon be 'migrating' to England this winter, it seems fitting to mention another group of migrants who are willing to pass up the warmth and culture of the Spanish Mediterranean for milky tea, warm beer, and Fray Bentos meat pies.

Every year, Blackcaps (Sylvia atricapilla) migrate from their summer breeding grounds in Central Europe to wintering grounds in southern Spain and Portugal. Over the past 40 years, a subpopulation of these birds has decided to go west instead, wintering in the much less pleasant British Isles. Since migration routes are genetically determined, this is a fairly radical change of direction which has raised numerous questions. Scientists want to know both why some of the birds are heading westwards (there must be some selective benefit, or this trait would die out of the population) and how can two genetically different populations maintain their distinctiveness if they breed in the same location?

The problem scientists encounter in studying this question is how to determine which birds came from where. These birds are too small for radio collars, and there are too many for mark and recapture studies. The genetic differences between the England winterers and the Spain winterers is probably extremely tiny, since the genetic change that led some to migrate west only turned up 40 years ago. This prevents any simple DNA tests to determine where the birds came from.

In a recent Science paper, Bearhop et al. make use of an original method to figure out where they birds come from... they measure the stable hydrogen isotope ratios in the birds tissues. The amount of deuterium (hydrogen with a neutron attached) to regular hydrogen (no neutron) in a bird is determined by the rainfall in the birds environment. Since England and Spain have very different rainfall profiles, Bearhop et al. were able to take small samples from birds as they arrived in Central Europe to breed and use the ratios to figure out whether the birds were Spanish or English.

What they found was that British birds arrived in Central Europe earlier than Spanish birds. When these birds went looking for partners, all they would find would be other Brits. By the time the Spanish birds arrived most of the English birds were already partnered up and the Spanish birds would naturally have Spanish partners.

The mechanism for this difference in arrival times is a very simple one... the birds use day length to tell them when to head for the breeding grounds. At northern lattitudes, the critical day length is reached earlier than at the more southerly lattitudes, causing the British birds to pack off to Europe 10 days earlier. The end result is two populations of birds breeding in the same location at more-or-less the same time but only breeding with their fellow countrymen despite the presence of other potential partners (a process called assortative mating).

So why go to England anyway? One obvious advantage is that by returning to Central Europe earlier, the British birds get first claim on breeding territories. Other possible advantages are the shorter distance that British birds have to fly, while the increase in backyard bird feeders in England may be aiding Blackcaps through the winter.

It will be interesting to know what happens to this population in the long run.... are we watching speciation in action, with the Blackcaps splitting into a British species and a Spanish species? Or are we seeing a selective sweep.... the eventual replacement of Spanish migrating Blackcaps with English migrating Blackcaps?

This will be one bird I'll be keeping my eye on when I have my own 'winter migration'.