Abstract Modern birds inhabit a remarkable breadth of ecological niches, a diversity that is in part attributable to the range of forms and functions that have evolved in the avian foot. Variation in the lengths, orientations, and internal proportions of their toes have enabled specialised grasping, climbing, and walking morphologies to evolve from a shared ancestral form. However, many individual elements of the foot are developmentally moderated in tandem. This integration of phenotypic traits can benefit the evolutionary process by coordinating traits to better maintain complex functions across generations, but may also inhibit adaptation in individual traits. Hence, trait integration has the capacity to constrain or enable evolution in certain directions. Here, we examine how adaptation for grasping (i.e., perching, climbing, and object manipulation) affects strength of integration in four common avian foot morphologies. Using measures of trait covariation and correlation, we conclude that grasping ability alone is likely an insufficient predictor of morphological integration. Overall, whole-foot integration strength and evolutionary potential are surprisingly consistent across the sampled morphologies, however, there exist differences in fine-scale integration patterns across species that merit further investigation. We find anisodactyl foot morphologies (e.g., fowl and songbirds) appear less integrated and more evolutionarily flexible than zygodactyl morphologies (e.g., parrots and woodpeckers), but that zygodactyl morphologies may be capable of a greater response to selection in certain directions. Such research helps to inform our understanding of both the evolutionary history of these taxa as well as how they may adapt to changing ecosystems in the future.
Martin et al. (Tue,) studied this question.