An experimental test for ecologically dependent reproductive isolation across an avian migratory divide

Divergent adaptation can promote ecological speciation if hybrids have
reduced fitness because they are poorly adapted to either parental niche.
We tested for ecologically dependent, postzygotic isolation between two
subspecies of Swainson’s thrushes, which form a migratory divide and
hybrid zone in western North America. To do this, we translocated
backcrossed and admixed birds from the hybrid zone into the range of each
subspecies at the beginning of fall migration. We estimated a proxy for
their survival on migration and migratory behaviour using automated radio
tracking. Apparent survival of birds in the two environments did not
depend on their genomic ancestry, suggesting that Swainson’s thrushes’
divergent adaptation to different fall migration routes does not fit the
classic model of ecological speciation. We propose an alternate scenario
where ecological selection on migration may interact with intrinsic
maladaptation in hybrids to produce reduced hybrid survival on migration.
By translocating birds from the same genomic backgrounds into different
environments, our experiment also allowed us to distinguish between the
effects of environmental relative to genetic contributors to their
migratory behaviour. We found evidence that both genetic and environmental
factors influence migratory behaviour, as the effect of genomic ancestry
on initial migratory trajectories depended on the start location for
migration, but birds ultimately followed expected routes given their
genomic ancestries.