I studied the recurrence of similar plumage-pattern elements in distantly related taxa across class Aves and examined whether such pattern elements are associated with morphological features, such as body size and anterior emphasis, and with habitat type and age class in selected examples. The identification of recurrent plumage-pattern elements across a wide array of taxa can be interpreted as evidence for parallel evolution and thus for shared developmental pathways. Developmental constraint, therefore, may serve to both generate and delimit the diversity of avian plumage patterns, which are subsequently presented to selection. By examining over 8300 examples of avian plumages mostly from the literature, I derived a plumage-pattern trajectory that can be used as a theoretical model to identify associated morphological trends within various taxonomic contexts of comparison. When comparing species within a clade, as body size increases, plumage pattern often shifts along a trajectory, from countershading through streaks, bars, uniform or bold black and white patches, to reverse countershading. Moreover, the trajectory is recursive at various taxonomic levels. The recursive nature of the trajectory, in which morphological and pattern elements recur in modified configurations, has perhaps contributed to an underestimate of parallel evolution in birds. I assert that a recognition of morphological recursion enables one to predict evolutionary trends in a hypothetical spectrum of potential form and plumage pattern and thus serves to define both the limits and possibilities of avian morphospace.
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Vol. 110 • No. 4