December 21, 2009—On a small rocky island off the Maine coast, two adversarial species of gulls coexist in relative peace, despite each being a potential predator of the other.
Thousands of Greater Black-backed and Herring gulls have evolved a complex system of social signals that keep violence on the island to a minimum. The gulls use these signals to carve out and hold small territories spread just 4.5 meters apart across the 95 acre island.
In their three short months on Appledore island, the gulls court, mate, nest, fledge young all within the same patch of rocks that they have held for years.
Territoriality begins when males arrive in spring followed closely by their mates. These experienced pairs of gulls strut about inspecting their site for appropriate nest locations and reinforcing territorial boundaries. Boundaries must be constantly defended from rivals. An interloper or loafer approaching a neighbors territory, is met with a series of displays or signals intended to drive him away.
Vocal signals include the long call, the mew, the kek kek and the yeow. The long call is a threat directed at 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 disturbed or in boundary disputes. The Yeow serves these same two functions.
Gulls also use body language to send signals in a visual display. An upright posture is rigid, neck stretched up and forward, head pointed slightly down. Wings are cocked forward, and slightly off the sides, poised to attack. Still more aggressive is the charge display. A ritualized attack that is symbolized by outstretched wings making the charging gull appear larger.
Fights happen in spite of the signaling system. But fights are risky, as severe injury to a parent will doom their eggs and chicks. Over evolutionary time, selective pressure on individuals has favored communication over conflict, resulting in the signaling system that reduces violence in the breeding colony.
Written by Bill Askenburg