Does the size and developmental stage of traits at fledging reflect juvenile flight ability among songbirds?

Abstract

Morphological traits can influence the ability of animals to perform ecologically relevant tasks that are essential for their growth, survival and fitness. Understanding such links in juvenile animals is of particular importance, as periods of juvenile life are often associated with high rates of mortality and can be critical with respect to evolution of life histories and viability of populations. To study such links, an important first step is to assess relationships between juvenile morphologies and measures of performance. Altricial songbirds can provide novel insights into morphology–performance relationships owing to the high locomotor demand placed on juveniles after they leave the nest. Therefore, we examined the relationships between the size and developmental stage of juvenile traits and juvenile flight ability among 13 coexisting songbirds in east-central Illinois. Among potential traits, we focused on relationships between flight ability and four measures of wing development at fledging as well as two body measures thought to reflect an individual's overall quality and musculature. Among measures of wing development, wing emergence was the strongest, most consistent predictor of ability within and among species. Relative wing length and wing emergence-loading were often good predictors of flight ability as well, while relative wing-loading was rarely a good predictor. Estimates of body mass and body condition were poor, inconsistent predictors of flight ability both within and among species. Our results highlight the importance of wing development for flight performance of juvenile songbirds. Furthermore, our evaluation of different wing characteristics suggests that the importance of macro-scale transmissivity and wing/disc loading towards flight ability changes with respect to juvenile ontogeny. Though mass likely interacts with wing characteristics in determining flight ability, our results suggest that body mass and condition are likely reflective of other, non-flight measures of fledgling performance. Few studies on altricial songbirds have examined morphology–performance relationships in juveniles. By linking wing emergence with juvenile flight ability, our research has taken a first step in assessing the adaptive significance of variation in wing development for juvenile songbirds. A free Plain Language Summary can be found within the Supporting Information of this article.