Effects of Human Intrusion on Song Occurrence and Singing Consistency in Subalpine Birds

Abstract

-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. Received 16 October 1992, accepted 28 January 1993. PRIMARY SONG is loud and far-reaching, occurring most often during the early breeding season and less frequently while young are being raised (Welty and Baptista 1988:224). Two important functions linked to primary song in breeding males are territory defense and mate attraction (Krebs 1977, Eriksson and Wallin 1986, McDonald 1989). Indirect evidence for the territorial function includes observations that most singing occurs during periods of territory establishment and maintenance and is restricted to defended areas (Falls 1988, Welty and Baptista 1988:227, 252). In addition, males countersing with conspecific males on adjoining territories (Wasserman 1977a), and they can be induced to sing with auditory playbacks that 4 Present address: John E. Conner Museum, Texas A&M University-Kingsville, Box 2172, Station 1, Kingsville, Texas 78363, USA. 5 Present address: 1306 Fox Street, Bossier City, Louisiana 71112, USA. simulate territorial intrusion (Falls 1981, 1988). The territory-defense role has been demonstrated experimentally through the surgical muting of males (Peek 1972a, Smith 1979, McDonald 1989), the tranquilizing of males (Peek 1972b), and the use of recorded playbacks (Krebs 1977, Falls 1988). The mate-attraction function of song has been corroborated with observations that singing activity is higher in males before they pair with females than after pairing (Wasserman 1977b). Experimental removal of females from pairs caused significantly higher singing rates by associated males (Wasserman 1977b, Krebs et al. 1981, Cuthill and Hindmarsh 1985). Moreover, males that were surgically muted were unable to attract females (e.g. McDonald 1989), even when such males possessed territories in good habitat and unmated females were present in the area (Peek 1972a). Because singing is important in territory defense and mate attraction, human disturbance that alters singing behavior may influence a