Timothy J. Mersmann’s research while affiliated with Virginia State University and other places

Although studies of bald eagle (Haliaeetus leucocephalus) food habits are numerous, few authors have quantified biases inherent in the techniques used. In our study of food habits of nonbreeding bald eagles on the northern Chesapeake Bay, we examined biases associated with pellet analysis, food remains analysis, and direct observation. We assessed these biases through controlled feedings of 2 captive bald eagles and through observations of free-ranging eagles. Fish fed to 2 captive bald eagles were underrepresented (P < 0.001) in egested pellets. Most bird and mammal food items were detected in ≥1 pellet; however, species and carcass condition affected frequency of occurrence. Analysis of captive eagles' food remains overrepresented birds, medium-sized mammals, and large bony fish (P < 0.001); small mammals and small fish were underrepresented (P < 0.001). This bias increased over time due to greater persistence of some remains in shoreline plots. Direct observations of free-ranging eagles resulted in biases toward easily identified species such as eels and catfish, but also documented the use of small, soft-bodied fish, which were not well documented by the other techniques. Because of the variety of biases present, accurate assessment of bald eagle foods requires use of multiple techniques.

A study was conducted to quantify diurnal perch habitat and examine the effects of time of day or season on the habitat selection of balt eagles (Haliacetus lencocephalus). An attempt was also made to test the hypothesis that perch habitat differed from habitat available at random on the Northern Chesapeake Bay. A total of 59 bald eagle were radio-tagged, and 220 bald eagle perch trees were identified. Eagles used a greater percent of dead or dead-topped perch trees than expected. No structural differences were found between perches used in May - October and perches used during November - April. These was also no time-of-day effects.

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.

We compared the distributions of resident breeding, resident nonbreeding, and northern and southern migrant Bald Eagles (Haliaeetus leucocephalus) on the northern Chesapeake Bay from 1984-1988. Breeding eagles were dispersed throughout most of the study area and were resident all year. Dispersion of Chesapeake nonbreeding eagles was similar to the dispersion of breeding birds on the northern Chesapeake in summer and winter. Chesapeake nonbreeding eagles moved throughout most of the bay, ≤5% of radio-tagged eagles were off the bay during any month. Radio-tagged northern migrants arrived in late fall (x̄ = 21 December, n = 7, range = 61 days) and departed in early spring (x̄ = 27 March, n = 14, range = 43 days). In contrast to local eagles, northern migrants were concentrated almost exclusively on Aberdeen Proving Ground, Maryland. Radio-tagged southern migrants arrived throughout April-August (x̄ = 6 June, n = 11, range = 94 days) and departed from June-October (x̄ = 3 September, n = 22, range = 119 days). Southern migrants were more dispersed than the northern migrants but less dispersed than the resident eagles. Northern Chesapeake eagle abundance peaked twice annually; in winter (e.g., 261 eagles, December 1987), due to the presence of northern eagles, and in summer (e.g., 604 eagles, August 1988), due to the presence of southern birds.

Only 55 of 1117 locations of radio-tagged Haliaeetus leucocephalus (4.9%) occurred in the developed land-cover type ≥4 buildings/4 ha), although 18.2% of potential eagle habitat was developed. Eagle use of the shoreline was inversely related to building density and directly related to the development set-back distance. Few eagles used shoreline segments with boats or pedestrians nearby. Only 360 of 2532 segments (14.2%) had neither human activity nor shoreline development. Eagle flush distances because of approaching boats were greater in winter than in summer (mean 264.9 vs. 175.5 m, respectively), but were similar for adult and immature eagles (203.7 vs. 228.6 m, respectively). Of 2472 km of shoreline on the N Chesapeake, 894 km (36.2%) appears to be too developed to be suitable for eagle use, and an additional 996 km (40.3%) had buildings within 500 m, thereby reducing eagle use. The projected increase in developed land in Maryland (74%) and Virginia (80%) from 1878 to 2020 is likely to determine the future of the bald eagle population in this area. (See also 91L/12673). -from Authors

We studied roosting behavior and habitat use of nonbreeding bald eagles (Haliaeetus leucocephalus) on the northern Chesapeake Bay during 1986-89. In summer and winter, 11 and 13 communal roosts, respectively, and many solitary roosts were used simultaneously in the $3,426-\text{km}^{2}$ study area. Radio-tagged eagles roosted solitarily with differing frequency by season (60, 21, 39, and 44% of 81 eagle nights in summer, fall, winter, and spring, respectively) (P < 0.05). Roost trees, predominantly oaks (Quercus spp.) or yellow poplars (Liriodendron tulipifera), were larger in diameter and provided greater canopy cover than random trees (P < 0.05). Roost sites had snags present more often than did random sites (P < 0.01). Most roosts (86%) were in woodlots >40 ha, and none were in human-developed habitat. In contrast, only 23% of random sites were in woodlots >40 ha, and 9% were in developed areas. Roosts were farther from human development than were random sites (P < 0.05); 57% of the roosts were found on public lands, compared to only 20% of the random sites (P < 0.001). Winter roost sites were protected from prevailing northerly winds more often than were summer sites (P < 0.05). We prescribe a 1,360-m-wide shoreline management zone that extends 1,400 m inland to encompass roost sites and provide a buffer from human disturbance.