The effects of water on patch use by two Simpson Desert granivores (Corvuscoronoides and Pseudomys hermannsburgensis)

Water loss while foraging may affect the overall value of food to desert an imals. When water is scarce, foragers may alter activity and shun certain t ypes of food due to elevated water loss. When water is abundant, foragers c an exploit food patches more thoroughly and remain active over a broader ra nge of ambient conditions. In short, food and water may be complementary re sources. The presence of water raises the marginal value of food, particula rly those foods low in water content. We tested for the complementarity of food and water to foragers at a sand dune site in the Simpson Desert of ari d Australia. To do so, we quantified patch exploitation of foragers in the presence or absence of bowls filled with water. In order to quantify patch use, we provisioned feeding trays with granulated peanuts mixed into a sand substrate. In these trays we measured giving-up densities (GUD; the amount of food left in a tray after a foraging bout) of diurnal (mostly Australia n ravens, Corvus coronoides) and nocturnal foragers (mostly sandy inland mo use, Pseudomys hermannsburgensis). The presence of water affected the GUD o f ravens but not of rodents. For the ravens, GUD dropped about 50% in respo nse to added water. For ravens, water and food are strongly complementary. In addition, ravens had lower GUD in the open than the bush microhabitat, a nd lower GUD at the bottom than the tops of sand dunes.