Great expectations: altricial developmental strategies are associated with more flexible evolution of limb skeleton proportions in birds

Birds have repeatedly evolved diverse developmental strategies, including multiple origins of sophisticated parental care, making them a model system to explore the consequences of developmental strategy upon phenotypic evolution. Here, we assess evolutionary covariance between limb proportions and ecological diversity of different bird lineages with altricial (high parental care) or precocial (lower parental care) developmental strategies. In addition, we model overall rates of evolutionary divergence between wing and leg skeletal proportions, allowing us to investigate the influence of developmental strategy upon adaptive traits. We show that while wing and leg proportions evolve independently of one another in altricial lineages, conforming to a modular pattern of evolution attested in birds more generally, there are strong correlations between wing and leg trait evolution in precocial lineages. Unlike precocial groups, divergent wing and leg evolution in altricial lineages is associated with access to novel flight-style combinations and is strongly associated with body mass. This suggests an adaptive association with mechanisms of growth and development. Inspection of internal wing proportions within major clades demonstrates that lineages with more altricial developmental strategies explore a wider range of mechanically relevant wing proportions, such as Brachial Index.