Fuel deposition rates of migrating birds may indicate the quality of habitat at stopover sites, yet little is known about how diet habits and food availability affect fat and protein metabolism in free-living songbirds at stopover sites. We compared plasma indicators of fat deposition (triglyceride), fat catabolism (B-hydroxybutyrate), and protein catabolism (uric acid) among passerine species that are frugivorous to a variable degree during autumn stopover on Block Island, Rhode Island. We also compared plasma lipid metabolites from 3 of these species that were captured at 2 stopover sites with different fruit abundance. The more frugivorous Hermit Thrushes (Catharus guttatus) had the highest plasma triglyceride, and uric acid was highest in the least frugivorous species sampled on Block Island, but other differences among species were not clearly related to diet. B-hydroxybutyrate was more variable among the species sampled on Block Island. Plasma triglyceride was significantly higher in Hermit Thrushes captured on Block Island, where fruit resources were abundant, than in Hermit Thrushes captured at a mainland site in southern Rhode Island, where less fruit was available. Our results suggest that diet habits may influence fat and protein metabolism in migrating passerines, but careful study design and statistical analyses are necessary to control for or minimize the effects of the many influential factors that affect plasma metabolites so they can be used to assess fuel deposition in free-living birds and to compare the quality of migration stopover sites.
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