Does foraging competition drive plumage convergence in three look-alike Atlantic Forest woodpecker species?

Abstract

Explaining the co-existence of sympatric and ecologically similar species is a central goal in ecology. In woodpeckers (Picidae), co-existence is frequently accompanied by plumage convergence. A particularly striking case concerns three woodpecker species in the Atlantic Forest of South America: Robust Woodpecker (Campephilus robustus), Lineated Woodpecker (Dryocopus lineatus) and Helmeted Woodpecker (Celeus galeatus), which show a remarkable degree of plumage similarity thought to result from convergence due to interspecific social dominance mimicry (ISDM). We studied the foraging ecology and interactions of these three species in old-growth forests and selectively logged forests to test the extent to which these species differ in various aspects of their foraging ecology, and we examined whether particular foraging requirements may help explain the association of the threatened Helmeted Woodpecker with mature forests. The species selected different tree species, tree diameters, foraging heights, decay states of trees and decay states of substrates, resulting in marked niche separation among the three species. Proportionally, the Robust Woodpecker chiselled more, Helmeted Woodpecker used more probing and Lineated Woodpecker used more hammering. Helmeted Woodpecker was the only species that included bamboos as foraging substrates, and it foraged more on dead wood than Lineated or Robust Woodpeckers, but mostly on small dead branches in live trees rather than standing dead trees. Foraging requirements are not the most likely factor explaining the association of Helmeted Woodpecker with mature forests. Limited resource and substrate overlap among the three woodpecker species, lack of interspecific interactions, and rarity of the Helmeted Woodpecker do not match predictions if these species were conforming to a mimicry complex under the hypothesis of ISDM. Instead, plumage convergence may aid in interactions with third species, or impart advantages in intraspecific competition.