Behavioral and physiological responses of Eared Grebes (Podiceps nigricollis) to variations in brine shrimp (Artemia franciscana) densities

Eared Grebes (Podiceps nigricollis) use the Great Salt Lake (GSL) primarily as a fall staging area. They feed on brine shrimp (Artemia franciscana) to build up energy reserves to complete their southward migration. Whereas it has been speculated that timing of grebe departure from the GSL is due to declining shrimp density, this hypothesis has not been tested. We monitored grebe weight, behavior, and departure dates to test this hypothesis and to determine the effects of declining shrimp densities on Eared Grebes during 1999, 2000, and 2001 when there were different shrimp densities. Despite differences in shrimp densities, grebes left the GSL about the same time each year. juvenile and subadult mass was significantly lighter (P < 0.001) in 1999 than in 2000 or 2001; however, adult grebe mass was similar in 1999 and 2000, despite the large difference in available of food. While there was no difference (P > 0.05) in dive or surface duration between 1999 and 2000, there were differences in the percentage of time spent foraging. Our results indicated that at shrimp densities < 0.1 shrimp (.) L-1, grebes increased their foraging effort to offset the declining food resource instead of departing the GSL at an earlier date.