Absence of nocturnal predator defense mechanisms in breeding gulls

Abstract

The theory that colonial, breeding gulls are well adapted to cope with predation pressures has become embedded in the behavioral and ecological literature. Under various circumstances, however, at least some gull species are incapable of effectively defending themselves or their young against predators, and as a result they may experience heavy losses. Nocturnal predation, in particular, poses a severe threat to some, and perhaps all, species of nesting gulls. It is, therefore, important to examine incidents of nocturnal predation on gulls and determine their compatibility with presently accepted theories. This thesis examines many such incidents in which nocturnal predators took a heavy toll on breeding gulls by killing adults and/or otherwise causing reproductive losses. My observations of nocturnal disturbances of Ring-billed Gulls (Larus delawarensis) and Herring Gulls (L. argentatus), along with a survey of the existing data, suggest that many gull species lack effective means of deterring predator attacks under the cover of darkness. Coloniality and other aspects of gull breeding biology that have been proposed as anti-predator devices in diurnal situations confer no advantage in nighttime confrontations. All gull species for which data were examined reacted non-aggressively toward nocturnal intruders. Many factors influence the precise form and frequency of these reactions, making accurate predictions regarding gull responses to predators difficult. The pattern of nocturnal predation and reaction of the gulls to it is contradictory to accepted views regarding the ability of gulls to cope with predation. The "anti-predator hypothesis" needs to be revised to recognize that many gull species have no means of defense against nocturnal invaders except selection of inaccessible colony sites.