greater black-backed and herring gulls breedon appledore island of the new england coast. here the gulls court, mate, nest, and fledgeyoung, all in three short months. but these two species are both predators. eating anything they can catch, kill, andtear apart, including other gulls. how can thousands of birds each a potentialpredator on the eggs and chicks of their neighbors coexist on a 95 acre island? the answer, these birds have evolved a complexsystem of social signals that keeps violence to a minimum.
the gulls use these signals to carve out andhold small territories spaced just 4 to 5 meters apart across the rocky island. territoriality begins when males arrive inspring followed closely by their mate returning to the same patch of rock they held in pastyears. these experienced pairs strut about. inspecting the site for appropriate nestingspots and reenforcing territorial boundaries. boundaries must be constantly defended. an interloper or neighbor approaching another'sterritory is met with a series of signals or displays intended to drive him away.
this is a long call. in a territorial context, a long call is athreat directed to trespassers. the mew, is also used in territorial disputes often calling in support from a mate. the kek kek is used when the colony is disturbedor in boundary disputes. the yeow, serves these same two functions. this visual display is an upright posture. the upright posture is rigid, neck stretchedup and forward, head pointed slightly down. wings are cocked forward and slightly offthe sides, poised to attack.
still more aggressive is the charged display. a ritualized attack that is amplified by outstretched wings, making the charging gull appear larger. fights happen, in spit of the signaling system. but fights are risky as severe injury to aparent will doom their eggs and chicks. over evolutionary time, selective pressureon individuals has favored communication over conflict resulting in a signaling system thatreduces violence in the breeding colony.