Status and Habitat Use by American Avocets Wintering at Humboldt Bay, California

Abstract

The wintering avocet (Recurvirostra americana) population at Humboldt Bay, California averaged 706 ± 35.6 (n = 30) and 567 ± 67.0 (n = 30) birds from October to May, 1982-1983 and 1983-1984 respectively. Avocets arrived from late-August to mid-November. Birds departed from February to late-April and early-May. Avocets are recorded only casually at other northern California coastal sites. After construction of sewage oxidation ponds at the northeast corner of North Humboldt Bay in 1957, the number of avocets wintering at Humboldt Bay increased. The present wintering population of avocets at Humboldt Bay has increased from 30-35 in 1960 to 500-800 at present. Humboldt Bay probably has the capability to support a larger wintering population than presently exists. Avocets used 868 ha in the northeast corner of North Humboldt Bay, California. Within this area, which covered approximately 25% of available habitat in North Bay, the four major habitats used were: (1) intertidal mud flats, used mainly for feeding and resting; (2) a sewage oxidation pond, used mainly for feeding and secondarily, as a source of fresh water; (3) a section of the North Bay's highest elevation mud flats, used as an early high tide roost; and (4) islands in a brackish 6.9 ha lake adjacent to the North Bay, used as the primary high tide roost. Avocets typically roosted on shallow, submerged bars of islands in deep non-tidal ponds or in shallow water or exposed mud near the water's edge of tidal mud flats. Islands adjacent to deep, non-tidal ponds were used most frequently as roosting sites. Avocets fed at the oxidation ponds mainly in October when high concentrations of invertebrate prey, especially Daphne magna, were present. All other foraging took place on intertidal mud flats within 3 km of roosts. Avocets primarily fed using tactile methods in the wettest substrates. They usually fed within 100 m of the tide edge, and most foraging took place when tide levels were between 0.5 and 1.2 m Mean Lower Low Water (MLLW).