The geography of risk: Motus telemetry reveals spatial variation in mortality of Setophaga striata (Blackpoll Warbler) during pre-breeding migration

Identifying when and where animals experience the highest mortality risk during migration is critical for understanding their long-term population dynamics. To estimate risk by latitude during pre-breeding (spring) migration for the steeply declining Setophaga striata (Blackpoll Warbler), we fitted 96 birds with radio transmitters on their nonbreeding grounds in Colombia and tracked their migration via the Motus Wildlife Tracking System. The resulting detection histories (82% of birds were detected at least once between Colombia and their boreal breeding grounds) were analyzed using a continuous-time multistate survival model, providing estimates of daily mortality risk in each of 4 regions (states) corresponding to discrete latitudinal bands. Hazard rates varied among regions, with the southeastern United States (including the Florida peninsula, ∼27–40°N) showing the highest instantaneous daily hazard (median = 0.027, 95% CRI: 0.001–0.143), ∼5 times higher than the region including northern Colombia and the Caribbean Sea (∼5–27°N; median = 0.006, 95% CRI: 0.000–0.024) and twice as high as the mid-latitude region (40°N to the southern edge of the breeding grounds; median = 0.012, 95% CRI: 0.000–0.064). Overall pre-breeding migration survival was estimated at 0.70 (95% CRI: 0.44–0.90), which is similar to estimates for other long-distance migratory birds. These results provide one of the first spatially explicit estimates of latitudinal mortality risk during migration and the first insights into regional variation in survival rates for S. striata. Our findings also highlighted the potential of Motus to be used as a tool for assessing survival in migratory species across broad spatial scales. Expanding tagging efforts and the coverage of the Motus array could provide much needed information on the vital rates of migratory species of conservation concern during vulnerable stages of their annual cycle.