Anthropogenic edges: Their influence on the American three-toed woodpecker ( Picoides dorsalis) foraging behaviour in managed boreal forests of Quebec

Abstract

We studied edge effects on the American three-toed woodpecker ( Picoides dorsalis) foraging behaviour in 18 remnant forest edges of black spruce and feather moss forests managed with a dispersed checkerboard pattern of clear-cuts. Our objectives were to assess (1) the characteristics of foraging substrates used by woodpeckers, (2) whether birds foraged according to the availability of high-quality foraging substrates found at varying distances from edges and (3) to characterize the movement patterns of foraging individuals near clear-cut boundaries. Behavioural observations of individuals allowed us to characterize all trees used for foraging according to their DBH, decay state, bark cover, tree species and top condition. We also georeferenced those trees, which allowed us to determine their distance from the edge and the orientation of the bird movements with regards to the edge. We sampled snags and downed woody debris along 80 m line transects that were oriented perpendicularly to the edge. Our results show that woodpeckers foraged in a relatively high proportion of live trees (35%). For live trees, woodpeckers used trees of larger diameter and black spruce was underused with regards to its availability. Among snags, woodpeckers preferred snags with a larger diameter, a lower decay class and a higher bark cover than nearest available snags. The density of high-quality foraging substrates (large recently dead trees) increased near the edge and decreased as we got farther into the forest interior. When comparing the distribution of used foraging snags with the one of available high-quality foraging substrates, our results show that these two distributions are significantly different. High-quality substrates located at 40 m or less from an edge were used less frequently than their availability. Hence, we can conclude that foraging woodpeckers can use snags near edges but are less prone to use these foraging trees even though they become more available than in the interior of remnant stands of managed forests. Nevertheless, bird movements were oriented parallel to the edge as far as 80 m away from the clear-cut boundary. Considering the under-utilisation of high-quality substrates near edges, we suggest that foraging substrate availability cannot explain the results obtained; the hypothesis that edges are acting as movement conduits likely explain woodpecker movement patterns we observed. Finally, the retention of larger tracks of mature and overmature forests would reduce the amount of edge habitat and provide better foraging conditions for American three-toed woodpecker in extensively managed landscapes.