COS 23-4 - Migratory birds with delayed spring departure migrate faster but pay the costs

Background/Question/Methods

Migratory birds that experience poor overwintering conditions are often late to arrive to the breeding grounds, which is known, in turn, to reduce individual fitness. Yet, despite the importance of this carry-over effect, few studies have investigated the extent to which individuals can modify migratory behaviors en route to reduce delays on arrival. Though birds might compensate for the late initiation of spring migration with a faster pace, trade-offs among time, energy, and safety suggest that accelerating migration incurs survival costs. For short-lived passerines with limited reproductive opportunities, the fitness advantages of catching up are likely to outweigh survival costs of attempting to compensate by migrating more quickly. To investigate these hypotheses, we used automated radio telemetry and light-level geolocators to track individual American redstarts (Setophaga ruticilla) as they migrated from wintering grounds in Southwest Jamaica through Florida en route to their breeding areas. Departure date alone is insufficient to determine whether an individual is delayed or not. As such, we leveraged long-term data on departure timing and breeding latitude to predict the expected departure date of tagged individuals. We then related relative migration timing (early vs. delayed) to individual migration rates and annual survival.

Results/Conclusions

First, using long-term departure timing data, we found that birds with more northern breeding origins initiated spring migration later than southern breeders, and males departed an average of 4 days earlier than females. Second, by using automated radio telemetry, we demonstrated that compared to those individuals that initiated migration earlier (4.5 km * hr^-1), individuals that departed relatively late (10-day delay), given their breeding latitude and sex, migrated at 42% faster rate (6.35 km * hr^-1). Independently supporting this pattern, we found the same pattern of delayed migrants migrating faster than early migrants for individuals tracked using light-level geolocators. Importantly, we demonstrate that redstarts with delayed spring departure and were expected to compensate with faster migration speeds were less likely to survive from one season to the next. In conclusion, our study is the first to provide evidence that migrants can compensate for delays in departure by accelerating the pace of spring migration, but that doing so incurs costs to survival. This compensatory behavior may therefore be a potential mechanism underlying differential survival during spring migration. Further work should explore potential constraints and tradeoffs between compensatory behaviors and life history attributes, migration rate, arrival timing, and reproductive success.