Foraging distribution and energy balance in wintering Dunlin

Kelly, John

This dissertation includes four chapters on the dynamics of foraging distribution and body mass regulation in wintering Dunlin ( Calidris alpina ) on Tomales Bay, California. The first chapter provides an overview of winter shorebird populations on Tomales Bay, based on ten years of baywide counts (n = 57) from 1989-90 to 1998-99. It examines tidally structured movements, intraseasonal and annual changes in species' local distributions, effects of rainfall, and annual population trends. It also identifies opportunities for habitat restoration and potential threats to foraging or roosting shorebirds, their habitats, and their prey. The second chapter focuses on the effects of weather and hydrographic conditions on foraging habitat selection in two wintering groups of Dunlin, which occupied different positions along a hydrographic gradient. Weather conditions, salinity, stream flow, and cumulative rainfall correlated with shifts in use of feeding areas. These patterns suggested that interactions between foraging requirements and hydrographic regime distinguish shorebird use among estuaries. The third chapter, co-authored by Nils Warnock, Gary Page, and Wesley Weathers, investigates the extent to which proximate cues related to the arrival and departure of winter storms influence the regulation of energy stores in Dunlin. Captive Dunlin fed ad libitum compensated daily for proximate conditions that were likely to predict changes in starvation risk. We found similar patterns in field data from Bolinas Lagoon, 20 km south of Tomales Bay. The final chapter, co-authored by Wesley Weathers, examines the effects of food availability and predictability on the regulation of body mass. Captive Dunlin under 24h:24h (ad lib:fasting) food restriction increased body mass relative to ad libitum fed controls, but 24h:6h:12h:6h (fasting:ad lib:fasting:ad lib)-restricted birds could not recover body mass lost during periods of enforced fasting. Body mass gain per g lost was greater in birds under restricted feeding than in birds under ad libitum feeding. Activity levels estimated from mass balance and thermostatic costs indicated that food-restricted birds reduced activity when feeding was possible, but increased activity when forced to fast. These patterns were consistent with the occurrence of non-migratory midwinter flights in free-living Dunlin, and endocrine-mediated changes in body mass and activity found in other bird species.