eWestern Birds

Historical Changes in the Abundance and Distribution of the American Avocet at the Northern Limit of its Winter Range

Mark A. Colwell, Tamar Danufsky, Ryan L. Mathis, Stanley W. Harris

ABSTRACT: Humboldt Bay, California, is the northern limit of the winter distribution of the American Avocet (Recurvirostra americana) on the Pacific coast. After the first record in 1935, avocets were uncommon (17 observations) until the early 1960s, when a wintering population of <100 birds became established in North (Arcata) Bay. Numbers increased to approximately 1000 by the early 1990s but have since declined to approximately 500. From 1968 to 1985, avocets consistently used intertidal habitats and oxidation ponds in the northeast quarter of Arcata Bay. Beginning in the mid-1990s they expanded their use of Arcata Bay and into South Bay. During February and March 1998 and February 2000, up to 32 occasionally fed in flooded pastures adjacent to the bay; only two avocets had been observed there in the previous 40 years. At low tide, avocets aggregated in intertidal habitats of Arcata Bay and South Bay. We hypothesize that they increased because of a rangewide population increase and that in Humboldt Bay they concentrate where small particle size of sediments makes for better feeding habitat. Altered habitat quality, especially during wet years (late 1990s), may have changed avocet distribution in Humboldt Bay.

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