Ecologic Overlap of Allen and Anna Hummingbirds Nesting at Santa Cruz, California

Abstract

If different species of birds or other animals overlap in their occupation of habitat, and if their requirements are more or less similar, there is reason to expect that they may compete. To study such competition in natural populations, it is necessary to gather evidence on aggression between members of the respective species, on their density levels and shifts in populations, and on related matters. These are rather large objectives considering how difficult it is to deal with such problems in the field. Nonetheless, it was thinking along these lines that motivated this study of nesting Allen and Anna hum-mingbirds (SeZusphorms s&n and Calypte anna) at Santa Cruz, California, from No-vember, 1953, through the breeding season of 1955. That there may be reduction of densities of breeding males of these two species in areas of overlap has already been reported (Pitelka, 195 1 b). For females, information is lacking. They nest in habitats different from those occupied by breeding males; and as no pair-bond is formed, nesting sites show no consistent relation to display territories of males (Pitelka, 1942). The nesting of the Allen Hummingbird has been studied in some detail (Aldrich, 1945), and the fact of overlap in the activity of nesting females of the two species is also known (Pitelka, 195 la). However, to date no serious attention has been given to the interrelations of nesting females on areas where both species occur. This paper is based on observations in such an area. Because the field work had to be terminated after two years, the results are only preliminary and suggestive. Although the main stress is placed on data concerning females, the territoriality, aggressive behavior , and relative abundance of males are reported when they add significantly to the local picture or to information available in the literature. All field work was carried out by Legg, and the first draft of this paper was prepared by him. The study was planned and carried out with suggestions from Pitelka, who then collaborated in the preparation of data and text for publication. The final versions of the maps shown in figure 1 were prepared by Mrs. Lois C. Stone, to whom the authors express their gratitude. STUDY AREA The study area, about six acres in size, included an entire small canyon located in the Natural Bridges State Park, Santa Cruz, California, at the north end of Monterey Bay. This canyon runs approximately north and south, and it varies in elevation from ten feet above sea level in the bottom to fifty feet at the rim. Both the north and south ends are bounded by city streets. The length of the canyon is 1200 feet. The area was divided into twelve equal sectors of 100 feet each to facilitate accurate plotting of nest sites. The width of the canyon increases gradually from 100 feet at the north end to 450 feet at the widest point (fig. 1). A winter stream carries runoff water from the hills to the north. This stream forms a pond at the south end of the canyon which gradually dries up in late summer and refills in winter. It always contains water through the breeding season. The south 1393 1 c