Increased sea-surface temperatures lead to increased egg cannibalism in Glaucous-winged Gulls (Larus glaucescens). Under these conditions, female gulls, which lay an egg approximately every 2 d, can synchronize egg laying with other females on an every-other-day schedule. Eggs that are laid synchronously are less likely to be cannibalized. The mechanism for synchronization has remained unknown. We studied the relationship between egg laying and mounting in a colony of Glaucous-winged Gulls on Protection Island, Washington, for clues to a synchronizing mechanism. We found that for an individual female an oviposition event reduced the likelihood of mounts for that day as compared to the day before, and that for sampled areas of the colony, total numbers of mounts and eggs laid occurred in an out-of-phase rhythm. These findings support the following conceptual model: early in the breeding season, individual females that have begun laying exhibit a daily alternation of higher and lower mounting activity caused by their natural 2 d oviposition oscillation. By social facilitation, these mounting oscillations of early layers synchronize mounting across the colony, including mounting events involving females that are not yet laying. Mount synchronization eventually leads to every-other-day egg laying synchronization.