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Summary1. 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.

SummaryStructurally-based ultraviolet (UV) coloration of plumage can signal male quality and plays a role in female mate choice in many bird species. UV-reflecting badges could also be important signals in male-male competition. We tested if territorial blue tit (Cyanistes caeruleus) males discriminate between conspecific male intruders which differ in the UV reflectance of their crown feathers. To this aim, we used a new experimental approach in which we simultaneously (instead of sequentially) introduced two male blue tit taxidermic mounts in the territories of resident males during the female fertile period; one mount with natural crown UV reflectance and one mount with reduced crown UV. The two mounts provoked strong aggressive reactions from resident males. Males specifically directed their aggression to conspecific intruders, as a male blue tit mount received substantially more aggression than a mount of a European robin (Erithacus rubecula). However, aggression of resident males did not vary between the UV-reduced and the control mount. Furthermore, the variation in natural crown UV reflectance of the resident males did not predict the intensity of their aggressive response. Contrary to previous findings our results suggest that UV signals play only a limited role in male-male interactions during territorial intrusions in the female fertile period.