Ecological speciation predicts that the fitness of hybrids will be
reduced if they exhibit intermediate phenotypes that fall between
parental niches. Empirical support for this prediction is sparse and
migratory divides may help fill this gap. Divides occur between
populations with divergent migratory routes. Hybrids in divides are
predicted to take intermediate routes over terrain avoided by pure
forms, reducing their fitness. We test this prediction here in a
well-characterized divide between Swainson's thrushes using niche models
and models of landscape connectivity. These models predicted lower
habitat suitability in the intermediate range between the migratory
ranges of pure forms and optimal routes that circumvent this area. Birds
that took intermediate routes used stopover sites of lower predicted
suitability and overlapped less with optimal paths than birds migrating
on either side of the divide. Our results have broad implications as
migratory divides are common in nature and not limited to birds.