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Metabolism and performance : a study of provisioning in the tree swallow, Tachycineta bicolor Burness, Gary P.


One goal of evolutionary physiology is to relate phenotypic variation to Darwinian fitness via organismal performance. Within this framework, I used breeding tree swallows, (Tachycineta bicolor) to identify physiological correlates and potential fitness consequences of inter-individual variation in parental energy expenditure (sustained metabolic rate, SusMR). I measured parental SusMR using the doubly labelled water technique and correlated it with variation in natural brood size and nestling growth rate and mass. SusMR was independent of natural brood size, although large broods had greater mass gain than small broods. I hypothesized that parental efficiency increases with brood size. Among adults rearing the same sized broods, SusMR increased with brood mass, and in one year, female SusMR and nestling growth rate were positively correlated. Natural selection is defined as correlation between variation in a phenotypic trait and variation in fitness. If nestling mass or growth rate are accurate indices of fitness, SusMR was under selection in this population. Individuals with high SusMR had relatively large intestines; presumably increasing digestive capacity. This may result in an increased resting metabolic rate and identify a potential energetic trade-off. I determined the influence of body composition on resting oxygen consumption rate (VO2). The mass of most organs differed between breeding seasons, possibly due to environmental conditions. Individuals with high resting VO2 had large kidneys but relatively small intestines. The basis of a negative relationship is unclear because the intestine contributes positively to VO2 in other species. A major determinant of parental life-time reproductive success is the survival of offspring to breeding. This is influenced by the quality of the rearing environment and its affect on offspring condition. Few studies have investigated what physiological and biochemical characters underlie variation in condition. I manipulated the number of nestlings in a brood and followed growth and resting VO2 until near fledging. Surprisingly, many characters were insensitive to environmental variation. Nonetheless, nestlings in reduced broods had a greater mass of lipid, increased cardiac enzyme activity, and higher size-specific resting VO2 than individuals raised in enlarged broods. How these characters affect survival or the future adult phenotype remains unknown.

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