Hybrid zone dynamics, assortative mating, and migratory programmes in a willow warbler migratory divide

Abstract: Popular Abstract in English In this thesis I will compare and contrast two willow warbler subspecies, each with different migratory programs that meet in Central Sweden forming a migratory divide and hybrid zone. These migratory programs differ in the direction and distance traveled during migration. The "northern" willow warblers migrate south-southeast through the Balkan Peninsula to winter in eastern Africa. The "southern" willow warbler migrates southwest through the Iberian Peninsula to winter in western Africa. In this thesis I will also explore the consequences of hybridization for these two very closely related subspecies where they meet in central Sweden. In the first paper I use long-term population monitoring data to see if the hybrid zone between the two subspecies shows lower abundance, suggesting that the hybrid offspring are less fit. Secondly, if there is lower abundance, is it associated with an environmental gradient likely to represent poor-quality habitat. In my second paper, I ask if there are population specific differences in their wintering moult ecology. To do this we look at feathers grown on the winter grounds that have isotopes reflecting their diet and habitat use. The third paper addresses the important question, does assortative mating lead to reproductive isolation or do these very similar subspecies hybridize and produce offspring? In my fourth paper, I ask does local adaptation to environmental conditions, such as temperature extremes and the short growing season, in mountain populations of willow warblers explain the apparent distribution of a genetic marker not related to the specific subspecies? Finally, in the fifth paper, I conduct a detailed analysis of traits from 50 sites across the hybrid zone, including 35 sites visited more than once. Here I ask, does lower abundances in the west of the hybrid zone predict the zone to be wider in the west than in the east? Further, using data from repeated visits to sites across the zone, we predict high annual turn-over in migratory phenotypes (or "migratype") occupying the zone. For future work directed at better understanding hybrid zone dynamics it will be essential to develop genetic markers that allow one to separate each parental migratypes, hybrids, and back-crosses. Once markers allow the identification of hybrid offspring, orientation experiments should be conducted to determine migratory directional preferences that would support our hypothesis that hybrids take an intermediate migratory direction to their parental migratypes. This intermediate direction could be a significant cost to hybrid fitness, as this route would require they cross the Mediterranean Sea and Sahara Desert at their widest points.