Habitat use and behavioral ecology of American avocets (Recurvirostra americana) wintering at Humboldt Bay, California

Population trends, habitat use, and behavioral ecology of American Avocets (Recurvirostra americana) wintering at Humboldt Bay, California were studied from 1 October 1982 to 15 May 1983, from 3 October 1983 to 8 May 1984 and from 19 December 1984 to 29 April 1985. Between November and February populations of American Avocets at Humboldt Bay averaged 706 ± 35.6 and 567 ± 67.0 birds in 1982-83 and 1983-84 respectively. Fall migrants began to arrive at Humboldt Bay in late August and by mid-November most of the wintering population was present. Spring migration began in February and ended in late April or early May. American Avocets used only 23.6 percent (868 ha.) of North Humboldt Bay. Within the northeast corner of Humboldt Bay 4 major habitats were used: (1) Intertidal mudflats, used mainly for feeding and resting, (2) Sewage Oxidation Pond, used mainly for feeding and secondarily as a source of fresh water, (3) Islands in Klopp Lake, used as principle high tide roosts and (4) a section of high elevation mudflat north of Jacoby Creek, used as an alternate high tide roost. American Avocets usually roosted in shallow water. Islands adjacent to deep, non-tidal ponds were important componets of roosting habitat. Use of roosts was tidally controlled. An average of 87.3 percent of all birds observed rested when tide levels were between 5.0 and 7.5 feet. American Avocets were the only common wintering shorebird to feed on the high concentrations of Daphne magna at the Oxidation Ponds. This food source was used mainly in October. Intertidal mudflats within 3 km. of the roost were the principle feeding areas during most of the winter. American Avocets used tactile feeding methods primarily and fed most often on the wettest substrates. Avocets usually concentrated near the tide edge when feeding. Greatest proportions of the flock was observed feeding when tide levels were between 1.6 and 4.0 feet. Most other shorebirds avoided American Avocets that were feeding. Interspecific and intraspecific competition probably was not ecologically important. Avocets foraged an average of 45-60 percent of the daylight hours in March and April and 84 percent between November and February. Avocets fed at night from November to March but not during October, April or May. American Avocets rested more on the intertidal mudflats and at the roosts during strong winds, during heavy rains, and when temperatures were less than 11°C than under other conditions. A greater proportion of Avocets fed on invertebrates found on exposed mudflats with visible surface water when temperatures were greater than 10°C than below 10°C. During light rains and under overcast skies American Avocets fed primarily on invertebrates found on flooded mudflats where the water covered their feet or on exposed mudflats with visible surface water.