Seasonal migration is a complex and variable behaviour with the potential to promote reproductive isolation. In Eurasian blackcaps ( Sylvia atricapilla ), a migratory divide in central Europe separating populations with southwest (SW) and southeast (SE) 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 14.0° E, 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.
Hybridization has important effects on the evolutionary trajectories of natural populations but estimates of this process in the wild and at the individual‐level are lacking. Justyn et al. attempted to fill this gap using the citizen science database eBird but there are limitations to this approach. Here, we outline and directly test these limitations using literature searches, case studies, and a comparison between eBird and Birds of North America (BNA), a database that documents hybridization using the scientific literature. We use a hybrid zone between Lazuli and Indigo buntings to highlight the importance of considering geographic range when estimating rates of hybridization and two literature searches to show the importance of considering cryptic hybrids (those that cannot be identified using phenotypic traits) when quantifying these rates. We also use BNA and a case study of hybrid White‐faced and Glossy Ibises to show that citizen scientists are underreporting hybrids compared with experts. Justyn et al. highlighted an important gap in the literature, but their results likely represent the lower limit of hybridization between birds and a more nuanced interpretation of their results (e.g., considering extrinsic postzygotic selection) is needed.
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.
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