Evidence for distinct evolutionary optima in the morphology of migratory and resident birds

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Abstract
Seasonal migration is prevalent in approximately one fifth of all bird species. Due to the high energetic costs of migration, consistent morphological patterns can be observed across migratory species. These include longer, more pointed wings and shorter tails in comparison to resident species. While evidence for distinct morphological adaptations of migratory species is well‐established, little is known about evolutionary trajectories of these morphological adaptations within groups of related species, and whether the evolution of these traits is consistent across different bird families. Here we apply a macroevolutionary approach to address this knowledge gap by comparing morphological traits and their evolution in migratory and resident species across eight families of passerine birds. We find a significant relationship of wing shape and tail length with migratory distance. When testing the evolution of wing shape and tail length in migratory and resident species, we find that a model with two distinct optima for migratory and resident species better explains the evolution of each morphological trait than a model with one evolutionary optimum. Thus, our results suggest consistent adaptive peaks in the evolution of these traits, and consistent selective pressures which improve the efficiency of long‐distance flight in migratory species and the efficiency of foraging flight in resident species. Our data provide a novel insight into the general patterns of morphological trait evolution in birds, thereby expanding the existing knowledge to a macroevolutionary scale in a field that has previously been dominated by species‐ or genera‐ specific studies.

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