EFFECTS OF HUMAN INTRUSION ON SONG OCCURRENCE AND SINGING CONSISTENCY IN SUB-ALPINE BIRDS

In 1989, 1990, and 1991, we conducted experiments on 30 circular 1.0-ha sites to assess whether human intrusions during a 10-week period influenced the occurrence and consistency of primary song in breeding subalpine birds. Using only those weekly censuses during which a species was present at a site, we computed song occurrence as the percentage of censuses during which a species sang, and we calculated singing consistency as the maximum number of consecutive censuses during which a species sang. An intrusion bout involved one person who walked through a site for 1 or 2 h. We used a priori contrasts, involving habitat covariates when appropriate, to assess differences in song occurrence and singing consistency between control and intruded sites and between sites at which the inner 25% of the site was disturbed (S25) and those at which 100% of the site was disturbed (S100). Singing by a number of species did not appear to be influenced by intrusion. For several species, however, song occurrence and singing consistency were higher on control sites than on intruded sites, indicating intrusion reduced singing activity. Song occurrence was higher on S100 relative to S25 sites as well. This latter pattern may have emerged because all of the individuals using the S100 sites were able to observe us during repeated intrusions and discern that we were not predators, whereas most of the individuals using the S25 sites likely did not have this opportunity. Thus, some of the individuals using S25 sites may have reduced their singing to avoid detection by us. Because song is essential in territory defense, mate acquisition, and in other reproductive activities, levels of intrusion that alter normal singing behavior have the potential to lower the reproductive fitness of males that are sensitive to this form of disturbance.