Reduced habitat suitability and landscape connectivity in a songbird migratory divide

Aim Seasonal migration is a common phenomenon in animals and connects geographically distant ecosystems. Considerable variation has been documented in this behaviour and migratory divides (contact zones between populations that use different routes to navigate around ecological barriers) are an example of this. Migratory divides could have important implications for ecological speciation as hybrids in divides take intermediate routes and it has been predicted that these routes will be ecologically inferior as they bring hybrids over barriers that parental forms avoid. We provide the first test of this prediction here using a well‐characterized divide between coastal and inland subspecies of Swainson’s thrushes. Location North, Central and South America. Time period 2006–2013. Major taxa studied Birds. Methods We used ecological niche models and analyses of landscape connectivity to predict suitable habitat for coastal and inland thrushes on migration and identify optimal migratory routes. We compared estimates of habitat suitability and landscape connectivity along the actual migratory routes of birds tracked with light‐level geolocators. Results Consistent with predictions for migratory divides, niche models estimated lower habitat suitability in the intermediate range between the migratory ranges of parental forms. Furthermore, our models of landscape connectivity predicted optimal routes that circumvent this intermediate area. Birds taking intermediate routes used stopover sites of lower predicted habitat suitability than birds migrating on either side of the divide and overlapped less with optimal paths. Main conclusions Our results support the prediction that intermediate routes of hybrids in migratory divides are ecologically inferior to those of parental forms. This finding suggests ecological differences are helping to maintain divides and could have broad implications, with divides shaping the distribution and maintenance of species boundaries globally and across many taxonomic groups.