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1 August 2002 CLAM DENSITY AND SCAUP FEEDING BEHAVIOR IN SAN PABLO BAY, CALIFORNIA
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Abstract

San Pablo Bay, in northern San Francisco Bay, California, is an important wintering area for Greater (Aythya marila) and Lesser Scaup (A. affinis). We investigated variation in foraging behavior of scaup among five sites in San Pablo Bay, and whether such variation was related to densities of their main potential prey, the clams Potamocorbula amurensis and Macoma balthica. Time-activity budgets showed that scaup spent most of their time sleeping at some sites, and both sleeping and feeding at other sites, with females feeding more than males. In the first half of the observation period (12 January–5 February 2000), percent time spent feeding increased with increasing density of P. amurensis, but decreased with increasing density of M. balthica (diet studies have shown that scaup ate mostly P. amurensis and little or no M. balthica). Densities of M. balthica stayed about the same between fall and spring benthic samples, while densities of P. amurensis declined dramatically at most sites. In the second half of the observation period (7 February–3 March 2000), percent time feeding was no longer strongly related to P. amurensis densities, and dive durations increased by 14%. These changes probably reflected declines of P. amurensis, perhaps as affected by scaup predation. The large area of potential feeding habitat, and alternative prey elsewhere in the estuary, might have resulted in the low correlations between scaup behavior and prey densities in San Pablo Bay. These low correlations made it difficult to identify specific areas of prey concentrations important to scaup.

Densidad de Almejas y Forma de Alimentación de Aythya marila y A. affinis en la Bahía de San Pablo, California

Resumen. La Bahía de San Pablo, al norte de la Bahía de San Francisco, California, es un área invernal importante para Aythya marila y A. affinis. Investigamos variaciones en el comportamiento de forrajeo de ambos patos entre cinco sitios en la Bahía de San Pablo, y analizamos si tales variaciones se relacionaron con las densidades de sus principales presas potenciales, las almejas Potamocorbula amurensis y Macoma balthica. Presupuestos de asignación de tiempo mostraron que ambas especies de patos pasaron la mayoría del tiempo durmiendo en ciertos sitios, tanto durmiendo como forrajeando en otros sitios, y que las hembras forrajearon más que los machos. Durante la primera mitad del período de observación (12 de enero–5 de febrero de 2000), el porcentaje del tiempo invertido forrajeando aumentó al aumentar la densidad de P. amurensis, pero disminuyó al aumentar la densidad de M. balthica. Otros estudios han encontrado que los patos se alimentan principalmente de P. amurensis y poco o nada de M. balthica. Las densidades de M. balthica en las muestras bentónicas fueron casi iguales entre el otoño y la primavera, pero las densidades de P. amurensis declinaron dramáticamente en la mayoría de los sitios. En la segunda mitad del período (7 febrero–3 marzo de 2000), el porcentaje de tiempo forrajeando no se relacionó fuertemente con las densidades de

Victoria K. Poulton, James R. Lovvorn, and John Y. Takekawa "CLAM DENSITY AND SCAUP FEEDING BEHAVIOR IN SAN PABLO BAY, CALIFORNIA," The Condor 104(3), 518-527, (1 August 2002). https://doi.org/10.1650/0010-5422(2002)104[0518:CDASFB]2.0.CO;2
Received: 23 August 2001; Accepted: 1 March 2002; Published: 1 August 2002
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