Reproduction of Ferruginous Hawks Exposed to Controlled Disturbance

Abstract

-The Ferruginous Hawk (Buteo regalis) is a conspicuous grassland bird that is sensitive to human disturbance. In 1978 and 1979, we studied 62 nesting pairs and recorded their behavior and nesting success. At 24 of these nests, we daily created disturbances designed to simulate those associated with land development on western rangelands. The other nests were not disturbed. Treated nests and control nests differed significantly (P < 0.05) in the number that successfully fledged young. Thirty-three percent of the disturbed nests were deserted by the adults, although our presence in the vicinity of the nest was brief. Those disturbed nests that were successful fledged significantly fewer young (P < 0.05) than undisturbed nests. Based on our cumulative data, adults did not flush 60% of the time if our activities were more than 120 m from the nest and 90% of the time if they were more than 250 m from it. Accordingly, for intermittent and brief human disturbance during years when prey are abundant and Ferruginous Hawks are in good physiological condition, we suggest that a minimum buffer zone of 0.25 km around the nest is sufficient to prevent nest desertion by at least 90% of the population. Buffer zone should be expanded in years when prey are scarce, i.e., when the hawks appear to be less tolerant of disturbance. The Ferruginous Hawk (Buteo regalis) is seriously affected in some areas by land development and, consequently some populations of the species are apparently small and may be declining. Reflecting this trend, the species has been on the Audubon Blue List since the list was established in 1971 (Tate and Tate 1982). A minimum number of breeding pairs was recently estimated at 2,810-3,590 over the entire range (M. Call, unpubl., U.S. Bureau of Land Management, Denver, CO, 1980). Ferruginous Hawks are particularly sensitive to human activity (Olendorff and Stoddard 1974, Fyfe and Olendorff 1976, Woffinden and Murphy 1977) and are prone to desert their nests if disturbed during incubation. Nonetheless, the species can respond well to management (White 1974, Porter and White 1976, Murphy 1978) so that it may be possible to adjust human activities to minimize ecological disturbance to it. Kennedy (1980) suggested such an approach in her assessment of conflicts concerning raptors and land use. It is becoming increasingly difficult to maintain critical wildlife habitat as the multiple-use demands on land intensify. The amount of disturbance that sensitive species will tolerate must be measured so that land management plans can be devised that are compatible with the species' needs. Our objectives were to determine how Ferruginous Hawks respond to disturbances, the levels of disturbances they will tolerate, and the size of buffer zones neede by disturbed pairs to maintain a level of nesti g success and productivity similar to those of undisturbed pairs. To accomplish this, we considered the natural tolerance thresholds of the species, the relationship of tolerance to prey availability, and the nesting history of the po ulation as related to disturbance. Most previous buffer zone recommendations from management agencies and the scientific community have been based primarily on best guess or anecdotal knowledge. Estimates of the sensitivity of Ferruginous Hawks, or raptors in general, to human disturbance were mainly obtained from observations made during studies of their breeding ecology. A significant exception is the Bald Eagle (Haliaeetus le cocephalus) for which considerable data were collected to aid development of sound management policies (cf. Grier 1969, Gerrard and Gerrard 1975, Mathison et al. 1977). Stalmaster and Newman (1978) examined the effects of some controlled disturbances on nonnesting Bald Eagles, but few other attempts hav been made to identify, quantify, and control variables that cause nest desertion. METHODS AND STUDY AREA We conducted our study throughout the Raft River Valley (42?N, 1130W), Cassia County, sou h-central Idaho (Fig. 1). The northern boundary of the valley opens onto the Snake