Cannibalism, the killing and eating of conspecifics of any life stage, leads to a variety of behavioral and ecological consequences and is influenced by a range of environmental circumstances among numerous taxa. In birds, cannibalism is particularly well known among gulls. Although multiple studies have linked cannibalism to egg and chick failure in gull populations, gull cannibal behavior is poorly understood. During the 2014 and 2015 breeding seasons, we observed the behavior of Glaucous-winged Gulls (Larus glaucescens) that used conspecific egg predation, a form of cannibalism, as a feeding tactic on Protection Island, Washington, USA. Most egg stealing by egg cannibals occurred during colony disturbances, especially by Bald Eagles (Haliaeetus leucocephalus), but egg stealing often occurred without concurrent disturbances. Among other findings, our data showed that egg cannibals were always males, the number of eggs stolen each day correlated with the daily number of eggs laid, egg cannibals ate fewer fish than non-cannibals, and the reproductive success of egg cannibals was lower than that of non-cannibals. This study provides the most comprehensive evaluation to date of how larid cannibals find, capture, and eat conspecific eggs, and it includes the first assessment of reproductive success of egg cannibals in comparison with non-cannibals.

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