Area sensitivity and edge avoidance: the case of the Three-toed Woodpecker ( Picoides tridactylus) in a managed forest

Abstract

Given the extensive progression of industrial forestry in boreal regions that reduces the area of old-growth forests and considerably increases the amount of sharp edges, important declines are to be expected among old-forest specialists area-sensitive or edge-avoiding boreal birds. The Three-toed Woodpecker ( Picoides tridactylus) is likely to be one of the species most negatively affected by boreal forestry and is possibly sensitive to forest area and edges. In this study, we quantify the area sensitivity and edge-avoidance of the Three-toed Woodpecker by analyzing its pattern of occurrence at 100 playback stations that had from 8 to 100% forest cover within a 300 m radius. Behavioral observations were also conducted to further investigate its response to edges in logged forests using foraging locations in relation to the nearest clear-cut border. Moreover, we document their foraging movement patterns in two contrasting landscapes (continuous versus shredded forests after logging). Although the occurrence of Three-toed Woodpeckers was highly related to the area of suitable habitat around the playback station, this relationship was quasi-linear and no critical threshold was found within the range of forest cover sampled. The amount of edge did not provide additional information on woodpecker occurrence. Individual woodpeckers in shredded forests did not select foraging trees further away from clear-cut edges than available ones. However, based on the results of the movement path analysis, continuous forests might provide better nesting habitat than residual, shredded forests. Indeed, spatial configuration of residual forest seemed to highly constrain foraging movements of this species because of its strong avoidance of open areas. Considering other studies conducted on forest birds, such modified movement patterns could be particularly harmful while both adults must feed their nestlings and behave as central place foragers. Therefore, even if no pattern of area-sensitivity or edge-avoidance were found, harmful consequences of forest shredding following forest logging may still occur for boreal species such as the Three-toed Woodpecker. However, residual forests strips are essential to maintain this species within managed areas, its population density within such residual forests being comparable to the one obtained in continuous forests.