Reduced hybrid survival in a migratory divide between songbirds

Migratory divides, hybrid zones between populations that use different seasonal migration routes, are hypothesized to contribute to speciation. Specifically, relative to parental species, hybrids at divides are predicted to exhibit intermediate migratory behavior and reduced fitness as a result. We provide the first direct test of the second prediction here with one of the largest existing avian tracking datasets, leveraging a divide between Swainson’s thrushes where the first prediction is supported. Using detection rates as a proxy for survival, our results supported the migratory divide hypothesis with lower survival rates for hybrids than parental forms. This finding was juvenile-specific (vs. adults), suggesting selection against hybrids is stronger earlier in life. Reduced hybrid survival was not explained by selection against intermediate phenotypes or negative interactions among phenotypes. Additional work connecting specific features of migration is needed, but these patterns p..., , , ## thrush\_juvenile\_morphology.csv

Morphological phenotypes from juvenile Swainson's thrushes.

Data were collected in the field from birds banded and tagged with individual tracking devices.
All birds were caught in British Columbia in a hybrid zone formed between inland and coastal subspecies of Swainson's Thrush.

* reference: individual ID associated with sequences uploaded to SRA
* release_site: description of site where bird was tagged and released
* release_year: year bird was caught
* tag_type: type of tracking tag. \"radio\" = motus radio tag
* sex_binary: 0 = male, 1 = female; based on Z and W sequences
* age_release: HY = hatch year
* release_gps.n: bird release latitude, in degrees north
* release_gps.w: bird release longitude, in degrees west
* motustagid: ID associated with individual tracking data, available at motus.org
* band: individual bird banding number
* tarsus.length: length of tarsus, in mm
* fat.score: visual assessment of fat, lower val...