Ecophysical constraints on avian adaptation and diversification

The evolution of morphological diversity is ultimately governed by physical laws and ecological contexts, which together impose a range of ecophysical constraints. Substantial progress has been made in identifying how these constraints shape the form and function of producers (plants), but similar knowledge is lacking for consumers, in part because the requisite data have not been available at sufficient scale for animals. Using morphometric measurements for all birds, we demonstrate that observed variation is restricted-both for beak shape and body shape-to triangular regions of morphospace with clearly defined boundaries and vertices (corners). By combining morphometric data with information on ecological and behavioral functions, we provide evidence that the extent of avian morphospace reflects a trade-off between three fundamental physical tasks for feeding (crush, engulf, and reach) that characterize resource acquisition and processing by the beak and three physical tasks (fly, swim, and walk) that characterize avian lifestyles or locomotion. Phylogenetic analyses suggest that trajectories of morphological evolution trend toward the vertices, with lineages evolving from a core of functional generalists toward more specialized physical tasks. We further propose that expansion beyond the current boundaries of morphospace is constrained by the shorter evolutionary lifespan of functional specialists, although patterns of speciation rate and current extinction risk provide only weak support for this hypothesis. Overall, we show that the structure of avian morphospace follows relatively simple rules defined by ecophysical constraints and trade-offs, shedding light on the processes shaping modern animal diversity and responses to environmental change.