Gap-crossing decisions by forest birds: an empirical basis for parameterizing spatially-explicit, individual-based models

Spatially-explicit, individual-based models are increasingly used to evaluate the effects of habitat loss and fragmentation on habitat use and population persistence. Yet, they are criticized on the basis that they rely on little empirical data, especially regarding decision rules of moving individuals. Here we report the results of an experiment measuring the gap-crossing decisions of forest birds attracted to a recording of chickadee (Poecile atricapillus) mobbing calls, and provided with options to travel to the speaker by either crossing an open area (short cut) or taking a longer route under forest cover (detour). We performed the experiment in winter and late summer near Quebec City, Quebec, Canada. We recorded 1078 travel paths from 6 resident and 12 migratory species in 249 experimental sites. In both seasons, birds preferred to travel under forest cover rather than cross open areas, even when the forested detour conveyed a substantially longer route than the short cut in the open. Only when the detour under forest cover. This was considerably longer than the short-cut in the open, in both relative and absolute terms, were birds more likely to take short cuts, indicating that gap-crossing decisions are scale dependent. However, birds rarely ventured > 25 m from forest edges despite having the opportunity to do so. Except for Hairy Woodpeckers (Picoides villosus) which ventured further into the open, all species showed similar gap-crossing decisions. Residents remained marginally closer to forest edges in late summer as compared to in winter. Conspecific group size had no influence on gap-crossing decisions. This experiment supports the hypothesis that forest bird movements are constrained in fragmented landscapes, and provides opportunities to calibrate spatially-explicit, individual-based models addressing the influence of landscape composition and configuration on dispersal.