The Effects of Forest Management and Food Availability on the Ecophysiology of Pre-Breeding and Breeding Hermit Thrushes (Catharus guttatus)

I investigated the physiology of Hermit Thrushes during the pre-breeding and breeding periods from 2002 through 2005 at the Penobscot Experimental Forest in Bradley, Maine. The objective was to understand how breeding birds meet their energetic requirements throughout breeding, and to evaluate impacts of forest management and food availability on energetics. Using hormonal, metabolic, and physical measures, I sampled the energetic condition of males during pre-breeding (Chapter 2), both sexes during breeding (Chapter 3), and both sexes provided supplemental food during breeding (Chapter 5). I also measured food availability within habitats under three forest management regimes (Chapter 4). Fat metabolism in adults and mass of fledglings suggested that the regenerating even-age habitats were of poorer quality for Hermit Thrushes than either shelterwood or selection cut plots. However, availability of the ground-dwelling invertebrates eaten by Hermit Thrushes was not found to vary markedly by forest management type, but there was higher abundance of most invertebrates in less shaded microhabitats, regardless of habitat type. Substantial seasonal decline in spiders may contribute to an overall decline in food availability through the breeding season. Variation in physiological condition suggested that fat metabolism is important to birds of both sexes during breeding stages in which individuals face food limitation. Prebreeding males carried fat stores that are hypothesized to aid in territorial defense and to assist in the transition to reproductive readiness. Incubating females also carried more fat than provisioning females, suggesting that food limitation associated with reproductive effort promotes prior fat storage. To experimentally test the hypothesis that food limitation influences energy balance in breeding birds, mealworms were supplemented to birds during the provisioning stage. Body mass and fat declined for both sexes over the provisioning stage and females carried more fat than males, but there was no effect of food supplementation on mass or fat. However, triglycerides were higher in females than males and higher in supplemented birds regardless of sex. This suggests that birds provided supplemental food were not fattening as much as control birds, since triglycerides are the primary pre-cursor of fat production. Corticosterone concentrations were higher in supplemented birds. Since this finding was contradictory to other studies, future studies should examine the relationship between food availability, corticosterone, and provisioning rates