Energetic costs and strategies of post juvenal molt in an equatorial bird, the rufous collared sparrow (zonotrichia capensis)

Abstract

Many tropical birds have slow‐paced life history strategies, exhibiting lower metabolic rates, reduced annual investment in reproduction, and longer lifespans relative to birds at higher latitudes. Life history strategies have been relatively well documented in adult individuals in the tropics, but we know comparatively little about the immature life history stage. Here we examine strategies of feather replacement (molt) and fattening in immature Rufous‐collared Sparrows (Zonotrichia capensis) in a high elevation equatorial population, following a parallel, previous study on an arctic congener, the White‐crowned Sparrow (Zonotrichia leucophrys gambelii). In captivity, Rufous-collared Sparrows incurred energetic costs of experimentally induced feather growth, similar to those previously described for Zonotrichia at higher latitudes. In contrast, free‐ranging immature Rufous‐collared Sparrows in natural molt had fat stores that declined over time, opposite to patterns evident in arctic Zonotrichia that fatten before migration. Equatorial birds in good condition molted more heavily (controlling for fat stores), suggesting that body condition limits the intensity of molt. Heavily molting equatorial sparrows also had lower amounts of fat (controlling for body condition), suggesting a trade‐off between allocation of resources to fat stores versus feather growth. Molt progressed slowly in Rufous‐collared Sparrows relative to previously described patterns in their arctic congener, which is concordant with a slower pace‐of‐life syndrome in tropical, as compared with high latitude, birds.