Oxidative stress, antioxidants, and patterns of frugivory in migratory songbirds

Oxidative stress is an unavoidable consequence of metabolism and increases during intensive exercise. This may be especially problematic for migratory birds that metabolize fat to fuel long-distance flight. In order to mitigate this burden, birds can either increase endogenous antioxidant production (e.g. uric acid) or increase consumption of dietary antioxidants (e.g. tocopherol). During flight, birds may strategically increase protein catabolism of lean tissue, which may increase circulating uric acid and overall antioxidant capacity. In addition, many birds consume a primarily frugivorous diet during autumn migration and many fruits are excellent sources of dietary antioxidants. The goal of my research was to investigate the roles of endogenous antioxidants and dietary antioxidants in mitigating the negative effects of oxidative stress in migratory songbirds. In my first experiment, I evaluated three related hypotheses: (1) protein consumption is positively related to circulating antioxidants in a migratory passerine, (2) a dietary oxidative stressor [i.e. polyunsaturated fatty acid (PUFA)] influences both antioxidant capacity and oxidative damage, and (3) oxidative stress influences the dietary antioxidant preferences of captive songbirds. White-throated Sparrows consuming a high protein diet increased circulating uric acid as predicted; however, uric acid, antioxidant capacity, and oxidative stress did not differ between birds consuming a high PUFA versus a low PUFA diet, despite increased oxidative damage in high PUFA birds. Birds did not prefer antioxidant-rich diets even when fed high PUFA, low protein diets.;In a second experiment, I evaluated whether fruit selection by birds at a stopover site in southern New England was related to the antioxidant and macronutrient content of wild fruits from seven fall fruiting shrub species. My specific objectives were to: (1) quantify, for the first time, total hydrophilic and lipophilic antioxidants, as well as two specific types of lipophilic antioxidants (i.e. carotenoids and tocopherols) in these wild fruits, (2) determine how the antioxidant content of wild fruits during autumn migration is related to macronutrient composition, and (3) relate patterns of avian frugivory to antioxidant availability and macronutrient content of wild fruits during the fall. There were significant differences between fruits in lipophilic antioxidants, carotenoids, and tocopherols, but not in hydrophilic antioxidants. Arrowwood (Viburnum spp.) and northern bayberry (Myrica pennsylvanica ) had the most lipophilic antioxidants and tocopherols whereas Asiatic bittersweet (Celastrus orbiculatus) and multiflora rose (Rosa multiflora ) contained the most carotenoids. Carotenoid content was positively correlated with protein content but no significant relationships were evident between the other antioxidants and macronutrients. Fruit consumption was negatively correlated with carotenoid content and was not related to any other antioxidant measure.;Taken together, these data suggest that: (1) birds are capable of mounting an endogenous antioxidant response to oxidative damage that is independent of uric acid, other circulating antioxidants, and dietary antioxidants and (2) though a variety of factors influence avian fruit selection during autumn migration, birds may be able to alter the quantities and types of antioxidants they consume on the fruits they select at stopover during autumn migration.