Individual Chick Recognition and Family Integrity in the Ring-Billed Gull

In field experiments designed to determine the nature and chronological development of chick recognition by natural-breeding Ring-billed Gulls, adults tending broods for a variable number of days were given recognition tests by replacing their own young with chicks from other nests. Introduced chicks of the same age as the resident brood were nearly always accepted during the first 5 days posthatching and usually rejected after the 7th day whether or not a resident chick remained on the territory. Adults with 7-day-old chicks presented with substitute 2-day-old broods either pecked or abandoned the younger chicks in almost all cases. Adults repeatedly presented with normally advancing-age substitute chicks each morning for the first eight days posthatching showed no signs of rejecting substitute chicks at any point. The pattern of parental activity shown by these birds resembled that of controls allowed to associate continuously with their own chicks. To examine the perceptual basis of recognition, a series of resident chicks 12-20 days old were surgically devocalized, and another series were marked with black ink to alter their individualistic plumage and facial patterns. No changes were observed in the behavior of parents toward muted or near-muted chicks, but over half of the parents attacked the visibly altered chicks. These attacks diminished within a few hours as the chicks persisted in showing positive approach responses to the attacking parent. In similar tests with 3- to 4-day-old chicks marked in the same way there were no rejections, suggesting that the markings did not, in themselves, constitute releasers for aggressive attack. The major findings of the study are interpreted as follows: a. Parent Ring-billed Gulls are able to recognize their own chicks after about 7 to 9 days posthatching, the stage at which increasing mobility terminates the family isolating mechanism provided by territorial boundaries. b. In identifying their own chicks, parent gulls can use individual variations in physical appearance. They may also respond positively to the relaxed comportment characteristic of chicks in familiar physical and social surroundings. c. In the social congestion that prevails in a gull breeding colony, individual recognition between parent and offspring provides the basis for a system of exclusive family units. This system, along with territorialism during the egg and early chick stages, permits a breeding bird to channel its reproductive energy to its own offspring exclusively, thereby promoting its own genes in contention with competing-gene carriers in the colony.