Physiological and behavioural correlates of life-history variation: A comparison between tropical and temperate zone House Wrens

1. We studied physiological, behavioural and demographic traits of House Wrens (Troglodytes aedon) in tropical Panama and temperate zone Ohio to explore the hypothesis that tropical birds with higher adult survival rates invest less in annual reproduction than their temperate zone counterparts. 2. Compared with wrens from Ohio, Panamanian wrens invested fewer resources in a given reproductive episode, as quantified by lower parental field metabolic rate (FMR) and water influx rate (WIR), a smaller number of feeding trips to the nest, and fewer chicks per brood. 3. Whole organism FMR and WIR were only 13-15% lower in tropical wrens, but because of their larger body size, mass-specific values were reduced by 34-37%. We propose that selection acts indirectly on whole-organism energy expenditure in the tropical wrens by specifically reducing mass-specific levels of metabolic rate, thereby postponing deleterious effects of aging and thus decreasing intrinsic mortality. 4. Declines in parental FMR and fledgling survival probability in the course of the season suggest that reproduction is more time-constrained in the temperate zone, compressed by the shorter breeding season, and as a result more energetically costly for the parents. In contrast, tropical wrens do not show temporal variation in FMR and nesting success and therefore are less constrained to breeding during a short season.