Winter Microclimate of Bald Eagle Roosts on the Northern Chesapeake Bay

Abstract

ABsTRAcr.-From 11 November 1988 to 1 April 1989, we studied the microclimate and nightly energy budgets at three separate Bald Eagle (Haliaeetus leucocephalus) communal roost sites; one used in winter, one used in summer, and another used year-round. We compared the roost sites with the microclimate and nightly energy budgets at randomly selected inland forested sites and eagle shoreline perch sites on the northern Chesapeake Bay. Mean and minimum air temperatures were similar among all sites. Mean and maximum wind speeds were greater at the shore than at other sites. Wind speed did not differ between roosts and inland sites. Among roost sites, mean and maximum wind speeds were lowest at the winter roost. The year-round roost and summer-roost winds did not differ. Mean net radiation was greater at inland sites than at the shore sites, whereas mean net radiation of roost and inland and of roost and shore sites did not differ. Minimum net radiation was greatest at inland sites, whereas roost and shore minimum net radiation did not differ. We calculated that roosting eagles would have expended 205.9 kcal per night at traditional roost sites, 206.6 kcal per night at shoreline perches, and 203.7 kcal per night at inland sites. Calculated energy expended on the 10 coldest nights was similar among roost, shoreline, and inland sites. Adding the estimated cost of transport from shoreline perches to roosts (x? = 5.3 kcal/round trip) did not produce significant differences in nightly energy expenditure between eagles roosting in communal roosts vs. those roosting on the shore. Received 7 June 1990, accepted 10 January 1991.