Energy content and digestibility of brine shrimp (Artemia franciscana) and other prey items of eared grebes (Podiceps nigricollis) on the Great Salt Lake, Utah

We measured the gross energy of brine shrimp (Artemia franciscana), brine shrimp cysts, brine flies (Ephydra spp.), brine fly larva, and corixids (Trichocorixa verticalis) from the Great Salt Lake (GSL). We estimated the apparent and true digestible energy in eared grebes (Podiceps nigricollis) for these prey items through feeding trials. We found no difference in energy concentration (kJ/g) between any of the invertebrates except for brine fly larva. Invertebrates from the GSL have a relatively high mean energy concentration ranging from 18.8 kJ/g to 23.9 kJ/g. Eared grebes are able to use 82.2–94.9% of the dry mass and 87.4–89.7% of the gross energy (true digestibility) content of their prey items. The high energy content and digestibility partially explain how an eared grebe can successfully forage on such small prey items on the GSL. In contrast, brine shrimp cysts had a high-energy concentration (23.5 kJ/g), but had the lowest amount of true digestible energy (34% of dry mass and 51% of energy), which helps explain why grebes are rarely observed foraging on them.