Nocturnal Behavior Reduces Predation Pressure on Black-vented Shearwaters Puffinus Opisthomelas

Many seabirds are nocturnal at their colonies, and it has been suggested that this behavior avoids diurnal predators.We examined nocturnal behavior in the Black-vented Shearwater Puffinus opisthomelas on Natividad Island, Mexico, to test the hypothesis that the current function of nocturnal attendance patterns is avoidance of predation by diurnal avian predators, primarily the Western Gull Larus occidentalis.We examined shearwater attendance and activity patterns throughout the breeding season, including activity on the colony surface, vocalizations of shearwaters and gulls, and proportion of shearwater burrows entered per night in relation to moon phase.We used direct observations and radio telemetry to examine the behavior patterns of shearwaters in flocks formed near shore at dusk.We found that shearwater activity was restricted mostly to moonless nights when light levels were below -2.2 log 10 lux(log 10 L).Fewer burrows were entered on nights with a full moon, when light levels were approximately -1.0 log 10 L, than on other nights.Predation rate by Western Gulls on shearwater models was highest during daytime (>0.0 log 10 L), intermediate during moonlight (-1.0 to -1.8 log 10 L), and lowest at complete dark (<-2.2 log 10 L).Our results support the hypothesis that a current function of nocturnal activity in the Black-vented Shearwater is avoidance of predation and indicate that an indirect effect of the behavior is reduced access to the colony.