A quantitative analysis of woodpecker drumming

The drumming patterns of 11 species of California woodpeckers were analyzed to determine if this long-distance signal is species-specific. Analyses for differences in the drum signal by sex, region, and playback initiation was performed on 5 of the 11 species. Nuttall's Woodpecker (Picoides nuttallii), White-headed Woodpecker (P. albolarvatus), Downy Woodpecker (P. pubescens), and Hairy Woodpecker (P. villosus) showed no significant difference in the drum signal for the above categories. Northern Flickers (Colaptes auratus) showed a significant difference in region and sex. Species discrimination showed signal overlap between Nuttall's and White-headed Woodpeckers, and between Hairy and Ladder-backed Woodpeckers (P. scalaris), and also between Northern Flicker and Hairy, Nuttall's, and White-headed Woodpeckers. Signal analysis of only syntopic species decreased misclassification and posterior probability error rates indicating syntopic, but not allotopic, signal specificity. Stepwise discrimination indicated cadence as the predominant variable for separation of species. Hairy and Downy Woodpeckers were not reciprocally misclassified, nor were Nuttall's and Ladder-backed, indicating drumming is a diagnostic indicator between these phenotypically similar species. We hypothesize that modulation of the drum in sapsucker species could minimize signal equivocation between syntopic woodpecker species with a matching introductory cadence. Signal equivocation between Northern Flickers and syntopic species is postulated as a possible selective pressure favoring the diagnostic long call prevalent in the species, rather than drumming.