The Eurasian Treecreeper (Certhia familiaris) has an effective camouflage against mammalian but not avian vision in boreal forests

Abstract

A well-known example of visual camouflage in birds is the plumage coloration of the Eurasian Treecreeper Certhia familiaris, yet this species’ camouflage has never been objectively quantified. Here, we quantify Treecreeper camouflage in its boreal forest habitat, test whether Treecreepers better match tree backgrounds at nest-site, territory or habitat spatial scales, and explore which common tree species provide the best background match. Using photographic data of the birds and forested backgrounds, we test their background match using human, ferret and avian vision modelling. We found that a Treecreeper’s wing and mantle provided the closest background matching, whereas the wing stripe and tail were more conspicuous against tree trunks. Vision modelling also suggests that Treecreeper camouflage provides a better protection against mammalian than against avian vision. The matching was not significantly different across spatial scales in local spruce forests. However, the background match was better on conifers than on deciduous trees. Our results support the long-standing conjecture of Treecreeper camouflage but also suggest that the camouflage is an adaptation especially against mammalian predators, which are important nest predators of Treecreepers.