Fat stores of American redstarts Setophaga ruticilla arriving at northerly breeding grounds
Many long-distance passerine migrants arrive with more fat stores than necessary to have reached their northerly breeding grounds. Researchers have argued for adaptive advantages associated with arriving with 'surplus' migratory fat, including increased breeding performance and insurance against adverse weather, and reduced food resources during the days following arrival. The objectives of this work were to document fat stores in American redstarts Setophaga ruticilla arriving to breed in northern Michigan and to test predictions associated with the hypothesis that arrival fat serves an insurance role. Results suggest that redstarts arrived in northern Michigan with fat stores sufficient to have continued migrating an additional distance greater than 1000 km. Significant yearly variation in arrival fat corresponded in part with environmental conditions measured at the breeding grounds. Birds arrived with the most fat in the year with the coolest temperatures and the lowest food abundance at the time of arrival to the site. Further, an inverse relationship between arrival fat and arrival day in males indicated that early arrivals carried more fat than later arrivals. Birds that arrived early faced cooler temperatures and lower densities of terrestrial invertebrates, and arrival fat may have provided a mechanism to overcome poor early season foraging conditions. However, our results are not entirely consistent with the hypothesis that arrival fat serves only as insurance. Arrival fat appeared important even during the most benign year of this study. Further, evidence suggests that females may have derived more benefit from arrival fat than males. These results highlight the connection between phases of a bird's annual cycle. Migrants that do well ell route, arrive at breeding grounds in better condition, which may contribute to survival and reproductive success.