Effects of Human Disturbance on Nesting of Bald Eagles

Known nests of bald eagles (Haliaeetus leucocephalus) on the Chippewa National Forest were divided into three groups reflecting degrees of isolation, called wilderness factors. The eagle nests under consideration were occupied 182 times from 1963-66. The rate of occupancy was essentially the same for each group. Nests in the high wilderness-factor category were successful 54 percent of the time, in the moderate category 57 percent, and in the low category 48 percent. None of these differences are statistically significant, indicating that human activity at levels existing on the Chippewa is not an important source of disturbance and has no measurable effect on nesting success or nest occupancy. Most human activity around nest sites in this region occurred during the latter part of the nesting cycle when family ties were strongest. Habitat modified by timber management in the immediate vicinity of nest sites did not appear to affect nesting activity. Failure of eagles to produce young on the Chippewa must be related to some factor other than human disturbance. The population decline of the bald eagle has been a subject of much concern since the studies of Broley (1947). Reported alterations in nesting success, of nest abandonment, and the disappearance of breeding populations in some areas (Robbins 1960) have caused conservation groups and agencies to study population trends and look for causes. The National Audubon Society is attempting to determine the status of the species on a continental scale and to study its basic breeding biology (Sprunt and Ligas 1966). The Bureau of Sport Fisheries and Wildlife is studying the relationship of insecticide contamination to reduced productivity (Stickel et al. 1966). Wintering populations have also been studied (Southern 1963, 1964 and others). My purpose here is to evaluate human disturbance as a potential cause of nesting failure among bald eagles on the Chippewa National Forest in north-central Minnesota. In 1963 the Eastern Region of the Forest Service, U. S. Department of Agriculture, established a policy for the protection and management of bald eagles and ospreys (Pandion haliaetus) on the national forests. One phase involves the establishment of buffer zones around nest sites. It is, therefore, a matter of some importance to investigate the tolerance of nesting eagles to human activity. An understanding of the effects of disturbance could either dictate more restrictive use of nest areas, or minimize the importance of disturbance as a factor limiting eagle populations. The breeding population of eagles on the Chippewa National Forest has been under study since 1963 (Mathisen 1963). As of May 1, 1967, there were 142 known nests on the Chippewa. For more detailed information on nesting studies see Mathisen (1966) and Grewe (1966). A summary of nesting activity from 1963-66 is given in Table 1. Average nesting success for the 4year period was about 47 percent. Roughly comparable data are presented by Sprunt and Ligas (1966) for other areas: Alaska 63 percent, British Columbia 73 percent,