Abstract
AbstractLaughing Gulls (Larus atricilla) were studied in a tidal salt marsh located in Brigantine, New Jersey and on an island in the Gulf of Mexico to determine how nesting gulls respond to conspecific intruders. Gulls were observed in Texas and at Brigantine during the pre-egg stage. In Texas, Laughing Gulls set up pairing territories in the sand beach area next to the colony, and defended these areas against conspecifics. In New Jersey, pairing territories were often located on Spartina mats within the colony area. Twelve pairs of gulls were marked, sexed, and observed for eight to fourteen hours per day during the incubation period. An intruder was defined as any gull other than the nesting pair which landed within three meters of the nest. The interactions between the nesting pair and intruders were recorded for 1145 sequences. Eighty-five percent of all intruders were responded to with the following patterns: Long Calls (18%), Intruder Displays (45%), and Chases (22%). Long Calls and Intruder Displays were given by an incubating bird, while Chasing was performed by the mate standing nearby. Gulls flying over the nest were usually reacted to with a Long Call although Intruder Displays were sometimes given. Intruders landing within one meter of the nest were always given an Intruder Display by the incubating bird and were frequently chased by its mate. Intruders landing from one to two meters from the nest were usually given the Intruder Display: those landing two to three meters from the nest were usually chased. Gulls landing over three meters from the nest were usually ignored. The probability that an intruder was responded to decreased as the distance from the nest increased. The behavior of intruders was as follows. Fifty-three percent of the intruders landing within a meter of the nests were rapists or potential rapists. These intruders remained on the nest longer than did other intruder, and sometimes evoked pecking and attack from the incubating bird. Nest-material stealers (18%) remained on the nest for less time. Neighbors (7%) infrequently landed on the wrong nest but quickly flew to their own nest. In 22 percent of the cases the activities of the intruders could not be determined, since they were quickly repelled.