Effects of food on bald eagle distribution and abundance on the northern Chesapeake Bay: an experimental approach

Availability of dead fish to bald eagles (Haliaeetus leucocephalus), prey preferences of bald eagles, and the effects of food on their distribution and movements on the northern Chesapeake Bay were examined from April 1988-July 1989. Dead fish surveys were conducted, by boat, to monitor dead fish availability in several eagle-use areas of the northern Bay, and 3 methods were used to describe disappearance rates of dead fish: dead fish cages, anchored dead fish and floating dead fish. Live fish availability was monitored using gillnets. Dead fish were most available to eagles from May through September, with a peak in availability in June (0.75 dead fish/km with fish die-offs not included, and 3.5 dead fish/km with fish die-offs included). Channel catfish (Ictalurus punctatus) comprised the largest portion of dead fish in early summer months (30% and 28% of total seen, excluding fish die-offs). In contrast, live catfish comprised only 0.4% and 2.1% of the fish caught near the surface in gillnets during spring and summer indicating that dead catfish may be more available, relative to other species, than live catfish. Atlantic menhaden (Brevoortia tyranus) comprised 83-98% of the fish seen in 2 fish die-offs (175 total fish). Only 2 dead fish were seen along 147.7 km of dead fish surveys in winter (0.014 dead fish/km). Most (95%) dead menhaden that we anchored near the bottom off Aberdeen Proving Ground (APG) in summer were scavenged before becoming rancid (X̅= 0.4 days). In contrast, 70% of dead menhaden that we put out in winter became rancid before being scavenged (X̅= 9 days).

Pairs of prey items were offered on shoreline areas to wild bald eagles and on platforms to 2 captive bald eagles. All pair-wise combinations of channel catfish, gizzard shad (Dorosoma cepedianum), menhaden and white perch (Morone americana) were offered. We also paired gizzard shad with mallards (Anas platyrhynchos) and rabbits (eastern cottontails, Sylvilagus floridanus, or domestic rabbits, Oryctolagus cuniculus) in shoreline trials, and gizzard shad with mallards and eastern gray squirrels (Sciurus carolinensis) in captive eagle trials. Wild and captive eagles preferred catfish (P=0.0072 and P<0.0002, respectively), and showed no preference for gizzard shad, menhaden nor white perch. Wild eagles preferred gizzard shad over mallards in summer and in winter (P= 0.062 and P=0.002, respectively), while captive eagles preferred mallards over gizzard shad (P= 0.039). Wild eagles selected gizzard shad 4 of 4 times over rabbits (P= 0.125), while captive eagles selected squirrels 5 of 5 times over gizzard shad (P=0.062, both eagles combined). Handling time and familiarity with prey seem to be major factors influencing prey preference, though prey availability seems to determine the actual diet of eagles on the northern Bay.

The prediction that the autumn decline in fish abundance on the northern Chesapeake Bay causes eagle distribution to shift from APG to 2 autumn/early winter concentration areas on the northern Bay (Susquehanna River and the Eastern Shore) and then to Blackwater National Wildlife Refuge (BWNWR) and vicinity (winter concentration area on the lower Bay) was tested. By supplying fish (mostly gizzard shad) ad libidum each morning at 2 sites from 28 September through 11 December 1988 a situation in which fish availability did not decline on APG was simulated. Eagle use of the sites increased from 4 eagles seen on first morning that we supplied fish to a peak of 63 eagles seen on the morning of 8 December. Based on shoreline surveys and relocations of 39 radio-tagged nonbreeding Chesapeake hatched eagles, eagle distribution shifted to the Susquehanna River, where eagles feed on gizzard shad, as in 1986 and 1987. However, they did not shift to the Eastern Shore to feed on waterfowl as they had done in 1986 and 1987. Supplemental feeding on APG failed to keep eagles from moving to the lower Bay. Although local eagle distribution on the northern Bay in autumn seems to be dependent on food availability, the autumn decline in fish abundance may not be the proximate factor causing movement to BWNWR and vicinity.