Individual variability and versatility in an eco-evolutionary model of avian migration

Seasonal migration is a complex and variable behavior with the potential
to promote reproductive isolation. In Eurasian blackcaps (Sylvia
atricapilla), a migratory divide in central Europe separating populations
with southwest and southeast autumn routes may facilitate isolation, and
individuals using new wintering areas in Britain show divergence from
Mediterranean winterers. We tracked 100 blackcaps in the wild to
characterize these strategies. Blackcaps to the west and east of the
divide used predominantly SW and SE directions, respectively, but close to
the contact zone many individuals took intermediate (S) routes. At 14.0ºE,
we documented a sharp transition from SW to SE migratory directions across
only 27 (10–86) km, implying a strong selection gradient across the
divide. Blackcaps wintering in Britain took northwesterly migration routes
from continental European breeding grounds. They originated from a
surprisingly extensive area, spanning 2000 km of the breeding range.
British winterers bred in sympatry with SW-bound migrants but arrived 9.8
days earlier on the breeding grounds, suggesting some potential for
assortative mating by timing. Overall, our data reveal complex variation
in songbird migration and suggest that selection can maintain variation in
migration direction across short distances while enabling the spread of a
novel strategy across a wide range.