| Summary: | Thesis (M.S.)--Humboldt State University, Wildlife Management, 1989 The species composition, distribution, and seasonal abundance of all waterbirds using South Humboldt Bay, California were studied from July 1987 through June 1988. Birds were recorded during direct ground counts and aerial surveys. Fifty-three species were seen during the surveys. An additional 38 species were recorded incidentally but were not present in sufficient numbers to warrant individual analysis. South Bay received 3,968,218 waterbird use-days in the year studied, about half of which were by roosting shorebirds. The numerically dominant shorebirds were peeps {Calidris spp.), marbled godwits (Limosa fedoa), dunlin {Calidris alpina), and willets (Catoptrophorus semipalmatus) . Waterfowl were the second most abundant bird group, accounting for 36.8 percent of the total bird use-days. Dabbling ducks, diving ducks, and black brant (Branta bernicla) constituted 44, 33, and 22 percent of the total waterfowl use-days, respectively. Principal species of dabbling and diving ducks were American wigeon {Anas americana) and American green-winged teal (Anas crecca carolinensis), and greater and lesser scaup {Aythya marila and affinis), bufflehead (Bucephala albeola), and surf scoter (Melanitta perspicillata), respectively. sixty-two percent of the total waterbird use occurred from midwinter to late spring. Spatial distribution varied with different species but, in general, dabbling ducks were found in greatest densities at the south end of the study area, while diving ducks, overwater piscivores, and herons had a more general distribution. Aerial estimates were compared to ground counts and provided generally poor correlations. For loons, grebes, and black brant, ground counts were higher than aerial counts while the reverse was true for diving ducks and American coots (Fulica americana) . Aerial estimates also indicated that, in general, waterfowl numbers were higher on South Humboldt Bay than on North (Arcata) Bay.
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