The blackcap (Sylvia atricapilla) is a warbler species common to breeding areas in central and northern Europe. Breeding populations of blackcaps in western Europe traditionally migrate south-west to wintering grounds in the Mediterranean and Africa. Recently, a fraction of blackcaps have established a new, north-western migratory direction and over-winter in the British Isles, resulting in a NW/SW migratory divide.
North of the Alps, an additional migratory divide is maintained by this species, in which autumn migrants orient either SE or SW to wintering grounds, but NW migrants have also been documented at this divide. Blackcaps from the NW-wintering subpopulation in Britain have been found to return to breeding grounds in southern Germany earlier than SW migrants from Spain. It has been suggested that this temporal difference in spring arrival to the breeding grounds may facilitate assortative mating in this species, in which blackcaps would prefer to mate with individuals from their same wintering grounds, and thus contribute to reproductive isolation between migrants from different winter origins. If barriers to reproduction persist in sypatrically-breeding blackcaps, these subpopulations may eventually evolve to become separate species. Although little is know about the about reproductive isolating mechanisms currently acting between these subpopulations, evidence has been found for restricted gene flow and phenotypic divergence among subpopulations of breeding blackcaps in southern Germany.
My research will investigate reproductive isolation among blackcaps belonging to different wintering subpopulations by collecting samples from individuals at both the NW/SW migratory divide in Germany and the SE/SW divide in Austria. Since migratory behaviour and direction has been shown to be genetically determined in this species, molecular analyses will be conducted to assign individuals to winter origin and detect genetic polymorphism, while morphologically characterization will further investigate phenotypic divergence between subpopulations.
Specific questions pertaining to this project include:
- What are the dynamics of the contemporary SW/SE migratory divide in Austria?
- Is selection against hybrids evident by comparing the two migratory divides?
- Is selection acting at the molecular level; for example, is there a presence of polymorphism in candidate genes or neutral loci?
- Is the novel NW migratory route repeated in Austria?
Comparing characteristics between two migratory divides will provide a better understanding of the microevolution of migratory behaviour, the potential for allochronic speciation, and help elucidate reproductive isolating mechanisms resulting from differing migratory strategies among blackcaps.