P-25 Experiments on the Effect of UV Reflectance on Egg Predation by Gulls

Conspecific egg predation (egg cannibalism) is common in gull colonies. Studies by our Seabird Ecology Team demonstrated that the rate of egg cannibalism by Glaucous-winged Gulls increases when food supplies are scarce, such as during El Niño-Southern Oscillation (ENSO) events. Gull retinas are sensitive to UV light reflectance. We hypothesized that UV light reflectance influences the selection of eggs by gulls for predation. We performed four experiments, each of which involved placing 50 white chicken eggs in randomly-placed artificial nests in the gull colony. Half the eggs were coated with a UV-blocking agent and the other half were coated with a control substance that did not block UV reflectance. The eggs were checked at 2-hour intervals throughout the day. Poisson regression showed that UV-blocked eggs were taken earlier than control eggs, although this effect disappeared on days with rain. Given that most gull eggs reflect little UV light, we concluded that gulls selectively preyed on chicken eggs that reflected light most like eggs of their own species, and that UV light reflectance plays a role in egg predation by gulls