PATTERNS UNDER SELECTION AND CONSTRAINT: EVOLUTION AND FUNCTION OF WOODPECKER PLUMAGE PATTERNS

Abstract

The beauty of complex spatial patterns in animals has long fascinated biologists, inspiring numerous hypotheses about the evolution and function of these patterns, from signaling and crypsis to thermoregulation and pest deterrence. The woodpecker family (Picidae) hosts an extraordinarily large number of species with diverse plumage patterns—from spots, to stripes, to bars, to streaks, to mottling— making this family an excellent study system for exploring hypotheses about pattern evolution. For example, plumage patterns in many woodpecker species have been suggested to provide crypsis against tree bark, the typical foraging substrate for these primarily scansorial birds (Thayer 1909). Furthermore, among Picids are several species pairs whose strikingly similar patterns may result from mimicry, as these species pairs are sympatric, size-dimorphic, and distantly related (Prum 2014). In this dissertation, I first explore woodpecker plumage evolution in the melanerpine (genera Melanerpes and Sphyrapicus) woodpecker clade in a comparative analysis that utilizes a novel pattern morphospace. Next, using a combination of field studies and museum specimen imaging, I test the hypothesis that New Jersey woodpeckers are more cryptic on trees they use for foraging than random trees from their habitat. Finally, I test for mimicry as a driver of plumage similarity in hairy (Leuconotopicus villosus) and downy (Dryobates pubescens) woodpeckers using behavioral data from bird-feeding stations. These investigations deepen our understanding of the diversity, evolution, and function of woodpecker plumage patterns and provide a novel framework for the study of complex spatial patterns in other animal taxa.