Quadrat Sampling of a Nesting Population of Bald Eagles

Abstract

An aerial stratified random sample survey of nesting bald eagles (Haliaeetus leucocephalus) was conducted during 1974 on 173,900 km2 of northwestern Ontario and southeastern Manitoba, Canada. I sampled less than the total (target) area to reduce cost and to avoid a search bias in areas previously visited. Fifty-three 100 km2 quadrats provided estimates of 428 ? 158 (37% ) eagles, excluding nestlings, and 291 ? 88 (30%) breeding areas for the sampled area and 579 ? 165 breeding areas for the entire area. Stratification reduced the variance of the means by roughly 22 percent for breeding areas in the entire area. A real visibility bias requires rigorously standardized techniques and, unless measured, limits the usefulness of estimates of actual numbers. The techniques should be feasible for a larger region and repeated sampling would provide a measure of trends in the status of this species of eagle. J. WILDL. MANAGE. 41(3):438-443 Claims that bald eagle populations have declined or not are based largely on reproductive information (Braun et al. 1975, Broley 1958, Gerrard 1973, Grier 1974, Laycock 1973, Sprunt 1966, 1969, Sprunt et al. 1973, U.S. Dept. of Interior 1974). The lack of simultaneous information on age-specific mortality and age of first breeding precludes the modeling of bald eagle population dynamics (Brown 1974, Brown and Cade 1972, Henny et al. 1970, Mertz 1971, Whitfield et al. 1974, Young 1968). Lack of adequate census data also hinders the determination of population status based upon changes in numbers of individuals. Counts of eagles on wintering areas (Fawks 1961 plus yearly updates, Hancock 1974, Higby 1975, Lint 1975, Southern 1963, 1964, Wrakestraw 1973) or during migrations (McClelland 1973, Spofford 1969, U.S. Dept. of Interior 1971) are of limited value because they reflect annual variations in weather and food availability. Furthermore, it is difficult to ascertain the source of eagles observed on migration or wintering areas. The National Audubon Society, U.S. Fish and Wildlife Service, and others have attempted to determine the total number of nesting bald eagles in the 48 contiguous states (Braun et al. 1975, Laycock 1973, Sprunt 1966, U.S. Dept. of Interior 1974). The majority of nesting birds, however, are found in Alaska and Canada, where only scattered attempts have been made to assess the eagle populations (Davis 1966, Gerrard 1973, Gerrard and Whitfield 1967, Grier 1967, 1969, King et al. 1972, Mansell 1965, Whitfield et al. 1974). All that is clear from the numbers of birds seen nesting in Alaska and Canada, and on wintering areas in the contiguous United States is that the present population of bald agles in North America must be relatively larg (Braun et al. 1975). Reliable censuses are needed to determine the status of the continental breeding population for management purposes. King et al. (1972) conducted a quadrat sample survey of bald eagles in southeastern Alaska. In searching for a technique to gain information on bald eagle numbers in Canada's boreal forest, in northwestern Ontario and southeastern Manitoba, I conducted a similar survey during 1974. The results of that survey as reported herein were encouraging and, with some modification, the techniques can provide useful trend information in future years. This research was supported by the Canadian Wildlife Service, Eastern Region, Project No. 6145/74-75. R. Buckler was my field assistant and pilot; without his valuable help the project would not have been con438 J. Wildl. Manage. 41(3):1977 This content downloaded from 207.46.13.112 on Mon, 03 Oct 2016 05:01:18 UTC All use subject to http://about.jstor.org/terms SAMPLING NESTING BALD EAGLES * Grier 439 Table 1. Classification and distribution of quadrats in the study area. Eleven quadrats with much human activity (towns, mines) and considered unsuitable for eagles were excluded