Woodpeckers are more cryptic against tree bark on which they forage

Abstract

Crypsis is a common strategy used by animals to avoid detection. In many bird taxa, plumage colour patterns are assumed to play a role in crypsis, but few studies have directly measured plumage crypsis on ecologically relevant substrates from the perspective of relevant signal receivers. To address this gap, we tested whether the plumage colour patterns of three North American woodpeckers commonly found in New Jersey (U.S.A.) are more cryptic to avian predators on trees utilized for foraging than a random subset of trees from their habitat. We focused on the red-bellied woodpecker, Melanerpes carolinus, downy woodpecker, Dryobates pubescens, and hairy woodpecker, Leuconotopicus villosus. First, we took standardized photographs of the bark of trees on which we observed woodpeckers foraging, as well as photographs of the bark of trees from transects in the same area. Next, we superimposed standardized photographs of woodpecker specimens (dorsal view) onto the bark samples and adjusted the resulting images to estimate how they would appear to the visual system of an avian predator. We then estimated four types of achromatic crypsis from each bird/bark combination image: granularity matching, contrast matching, luminance matching and disruptive coloration. We also estimated how well woodpeckers matched the bark substrates in terms of colour by quantifying the degree of colour similarity in a colour space representing a typical avian predator. We found that woodpeckers were more cryptic against trees they utilized for foraging in terms of disruptive coloration, luminance matching and colour matching, suggesting that the need to be cryptic while foraging is likely to have shaped the evolution of woodpecker plumage patterning.