BIRD TOLERANCE TO HUMAN INTRUSION IN WYOMING MONTANE FORESTS

Human intrusion can be a serious problem for birds because it can caus e displacement, prevent access to resources, and reduce reproduction a nd survival. The factors that influence avian tolerance to intrusion a re poorly understood. We studied passerine responses to intrusion in W yoming montane forests during the breeding season by using two indices of intrusion tolerance: detectability period, the amount of time that a bird remains near its initial flush point; and approach distance, h ow close one can get to a bird before it flushes. A solitary observer experimentally approached focal individuals and recorded detectability period, approach distance, the seasonal and daily timing of intrusion , number of nearby conspecifics, number of nearby heterospecific indiv iduals, and surrounding vegetation conditions. Using data from the lit erature, we also assessed influences of migratory status, body mass, c onspicuousness, and height above the ground at which species are activ e during the breeding season. Detectability period was significantly s horter, indicating intrusion tolerance was lower, when fewer conspecif ics were nearby. Approach distance was significantly longer indicating tolerance was lower, for more-conspicuous species and for species tha t are active closer to the ground. Effects of other variables studied were not significant. These results demonstrate that social and biolog ical factors can influence tolerance to intrusion. Intrusion-induced b ehaviors such as nest abandonment and decreased nest attentiveness hav e led to reduced reproduction and survival in species that are intoler ant of intrusion. With knowledge of factors that influence tolerance, the risk of disturbing birds that are sensitive to intrusion could be reduced.