An Aerial Census of Bald Eagles in Saskatchewan

Abstract

An aerial census was conducted to determine the numbers and productivity, and to delineate distribution of bald eagles (Haliaeetus leucocephalus) in a 270,000-km2 area of northern Saskatchewan. Randomly located sample units were designed to contain uniform amounts of primary nesting habitat. An estimated 3,900 breeding areas, 6,900 adults, 4,700 immatures, and 2,500 nestlings were present in the summer of 1974. In southwestern regions 0.95 young per breeding area were produced, indicating reproduction above the estimated minimum required for maintenance. There was a positive correlation between breeding area density and April mean temperature. Two models for eagle distribution in the province are discussed. As a monitoring tool, the census technique could detect a population change of 27%. J. WILDL. MANAGE. 43(1):61-69 In 1969 Whitfield et al. (1974) censused the bald eagle population in approximately 20% of the boreal forest of central Saskatchewan and Manitoba. They estimated 460 to 697 breeding areas and a fall population of 1,600 to 8,000 eagles in the most densely inhabited part of their study area. Their results showed that these provinces are major inland bald eagle breeding grounds in North America. The study reported here was undertaken to estimate the size, reproductive success, and pattern of distribution of the bald eagle population in Saskatchewan in the summer of 1974, and to develop an aerial census technique suitable for monitoring the population. We are grateful to 3 agencies for financial support: Canadian Wildlife Service; Churchill River Study, Wildlife Sector (CRSWS); and Parks Canada. The CRSWS also made data available to us. We acknowledge the criticism and advice of R. W. Longley on meterological interpretations, E. Nordbrock on statistical procedures, and J. B. Gollop who reviewed the manuscript. MATERIALS AND METHODS Census Area and Sample Units The 270,000-km2 census area includes most of the boreal forest of Saskatchewan from approximately Lat. 54'N to the province's northern boundary at Lat. 60'N (Fig. 1). The area's southern boundary was chosen to eliminate from the census that portion of the province which we believed from previous work to have few breeding bald eagles (Whitfield et al. 1974). All of the census area except a strip of varying width along the southern edge is on the Precambrian Shield. The shield is covered with numerous lakes and ponds and its boundary with the more southerly sedimentary rock is marked by a series of large lakes which straddles the zone of contact. Relief is generally low and drainage is into Hudson Bay and the Arctic Ocean. The major physiographic regions are the Precambrian Shield, including the Athabasca Plains and Rock Knob Complex, and the Central Lowlands south of the shield (Richards and Fung 1969). J. Wildl. Manage. 43(1):1979 61 This content downloaded from 207.46.13.73 on Sat, 14 May 2016 06:08:52 UTC All use subject to http://about.jstor.org/terms 62 BALD EAGLES IN SASKATCHEWAN* Leighton et al.