Assortative mating and microhabitat choice of blackcaps (Sylvia atricapilla) along a migratory divide
Since the 1960s, a new overwintering strategy has been developing in the blackcap: A growing part of the breeding population in southern Germany does no longer migrate to the traditional overwintering regions on the Iberian Peninsula, but orients instead to the northwest, to the British Isles. Previous studies showed that birds overwintering in Britain gain fitness benefits and arrive significantly earlier on the sympatric breeding grounds in central Europe than the southwest migrants.
As the migratory direction has shown to be heritable in blackcaps, assortative mating, so to say “British” blackcaps mating with “British” partners and “Spanish” blackcaps with “Spanish” partners, should be advantageous since it reduces the risk of producing unfit offspring migrating to an intermediate direction. Morphological differences between these two subpopulations (e. g. NW-migrating blackcaps are known to have rounder wings and a longer beak than SW-migrants) make it possible to phenotypically distinguish between a NW-migrant and a SW-migrant, which is an important prerequisite for assortative mating. Therefore, barriers to gene flow are possible to arise which may lead to reproductive isolation of the two ecotypes and eventually to the evolution of two different species.
In my diploma thesis, I am testing the hypothesis whether blackcaps pair assortatively according to their migration behavior. To achieve this, I catch blackcaps upon arrival to the breeding grounds in the Mooswald near Freiburg and mark every bird with an individual combination of colored rings. By re-sighting and observing the birds, I try to figure out which two individuals form a breeding pair.
The questions I will ask in my work are the following:
Do breeding partners have the same wintering grounds?
I will answer this question via analysis of stable hydrogen isotopes in claw tips, which allows assessing the latitude of the respective wintering grounds.
Is there a similarity in morphology between the paired birds? Is this similarity stronger between the two partners than to random birds of the whole population?
By comparing morphological traits like wing shape or length of tail, tarsus, and feathers, I will be able to analyse the extent of assortative mating.
Is there a difference in microhabitat choice of NW- and SW-males?
If this is the case, there would be an additional behavioral barrier to random mating. To test this hypothesis, I will examine habitat variables in the different territories, like vegetation covering in different layers, availability of food plants or proximity to major roads and other disturbances, and check whether habitat preference correlates with the migratory behavior of the male choosing that territory.