Life Science Mass Spectrometry Facility

 Segregation of breeding Blackcaps with respect to wintering area and its potential consequences

The last 30-40 years has seen a dramatic increase in the number of Blackcaps overwintering in the British Isles. Work on captive individuals has shown that these birds breed in southern central Europe (this population normally winters in Iberia & North Africa) and that the shift has a genetic basis.

The rate of increase suggests that selection may favour this migratory shift. Putative fitness benefits include: early arrival on the breeding grounds & access to best territories. Assortative mating has also been predicted, due to differences in arrival times between southern and northern winterers. Also the young of mixed pairs orient in an intermediate migratory direction which potentially takes them into less suitable wintering areas.

We can use claw dD (=d2H) ratios to infer wintering latitude of birds on the breeding grounds. Rainfall dD ratios are taken up by plants and reflected throughout the foodweb in a predictable manner, such that resident birds in the British Isles have different claw dD signatures from those living in southern Europe, since dD becomes more negative with increasing latitude (Fig. 1). As claws are metabolically inert, the winter habitat signature is preserved indefinitely. Thus clipping a small piece of claw from a blackcap during the breeding season can allow us to infer the latitude at which it spent the winter. We have shown that the inferred dD patterns are recorded in overwintering blackcaps, which enables us to test some of the ideas above.

We found that there was an association between the dD values in the claws of males and females that they were paired with, indicating that they mate assortatively with respect to their wintering area. Birds with claw dD ratios indicating more northerly wintering areas arrived earlier than those from further south suggesting that temporal differences in arrival times may be the force that drives assortative mating. We also discovered that birds adopting the new wintering strategy tended to have better breeding success than those from Iberia. These findings have two crucial implications. First, they indicate the mode in which sub-populations can become isolated in sympatry which can be the first stage of divergence and ultimately speciation. Second, since climate amelioration has been implicated as one of the forces driving migratory change our results indicate one way in which migratory species may be able to respond.

This work has been recently published in Science.

For more details contact Stuart Bearhop

Blackcap picture kindly reproduced with the permission of BirdLife Cyprus