Crossing the Straits of Mackinac: over-water uplift, wind support and low wind speed facilitates water crossings by a soaring migrant during pre-breeding spring migration.

Abstract

Background: Large bodies of water are ecological barriers known to influence the migration properties of avian migrants. Among soaring raptors, various flight strategies are used during migration to overcome these obstacles. These raptors exploit favourable winds and thermals to successfully migrate since prolonged flapping flight is expensive energetically. Soaring migrants circumvent large expanses of water and cross at geographical bottlenecks, minimizing the amount of open water they cross to save energy. Few studies have directly investigated how atmospheric conditions influence water crossing behavior during northbound pre-breeding spring migration at a high concentration bottleneck.However,inefficientlycrossingecologicalbarriersduringspringmigrationcanleadtodelayedbreedingandfitness consequences for soaring raptors.

Methods: We use the Red-tailed Hawk (Buteo jamaicensis) as a model organism to investigate how atmospheric conditions influence the timing of water crossing at the Straits of Mackinac in Michigan, USA. We deployed 36 GPS transmitters during spring migrations spanning 2021-2023. Fifty-one water crossings from 33 adult individuals were used in this analysis. We annotated crossing events with atmospheric covariates including wind speed, wind support (V), crosswind (U), and uplift potential (temperature difference between water surface and air) to elucidate what conditions explain the probability of water crossing behavior by Red-tailed Hawks.

Results: We found that uplift over water was available in spring and that it positively affected the probability to cross. We also found that wind support and wind speed had a significant effect on crossing probability as low wind speeds and supportive wind increased the probability of crossing. The hawks had a higher probability to cross earlier in spring.

Conclusion: We provide evidence that uplift over water along with low overall wind speed and wind support facilitate water crossing behavior of Red-tailed hawks during pre-breeding spring migration. We also highlight that despite the general effects of uplift and wind there is substantial variation in crossing behavior among individuals. Our findings further contribute to the understanding of how soaring raptors overcome water barriers during migration.