Communal Social Biology of the Southern San Blas Jay

Abstract

We studied the communally-breeding Southern San Blas Jay, Cyanocorax (Cissilopha) s. sanblasianus from 1974 to 1978 near Acapulco, Guerrero, Mexico. In this area it inhabits only palm plantations and the small remnants of native woodland scattered among them. Most nests are built in the crowns of coconut palms.

This jay lives year-round in nonmigratory groups of fairly constant composition, which on our study areas ranged in size from 13 to 26 birds one year old and older. In each group 62-82% of the members were at least 3 years old. Each group contained 6-10 breeding pairs that were probably permanently monogamous, plus a smaller number of nonbreeders. With the exception of replacement nests and the rare occurrence of second broods, female breeders laid in only one nest per breeding season, and only one female laid in each nest.

Breeding pairs were assisted in feeding and defending their young by both the nonbreeders and breeders of their communal group. Some nests were attended by as many as 13 birds. Helpers. whether breeders or nonbreeders, preferred to feed fledglings rather than nestlings. Nests begun earlier in the breeding season attracted more helpers than did later ones, because of the accumulation of fledglings as the season progressed.

The feeding rate per nest increased with brood size, but there was no correlation between the feeding rate per nestling and brood size. The positive correlation between feeding rate per nest and brood size is probably a result not of greater rates of feeding per attendant, but of larger numbers of attendants, each of which feeds at a rate independent of the number of nestlings. All birds of known parentage helped at their parents' nests, though none helped its parents exclusively.

Most birds began breeding at 3 years of age or older, but 1- and 2-year-olds were breeders more commonly than is at present evident in the other three species of Cissilopha. Younger breeders tended to nest later in the breeding season than older breeders.

Each communal group inhabited an exclusive home range, whose boundaries with other groups were maintained by mutual avoidance rather than by active defense. In one large group, whose members were dispersed over a comparatively large home range, breeders with active nests confined their activities to a small fraction of their group's home range and exerted a quasi-territorial dominance over other group members who entered these areas. Such individual "core areas" were not evident in smaller groups.

The commonest clutch sizes were 3 and 4. Incubation lasted 17-18 days, and the nestling period was about 18-20 days. Breeding pairs were usually single-brooded. Between 50 and 68% o f nests that had clutches produced at least one fledgling, and 22-38% of all eggs resulted in fledglings. From 0 to 50% of young that fledged survived to at least 1 year of age. The annual survival rate of yearling and older birds was about 75% in most years. Changes in group membership, by both young and adults, appear to be of infrequent but regular occurrence. Females seemed to change groups more often than did males. Most of the immigrants to our study groups were of unknown origin.

The movements of dependent juvenile and their attendants are described.

Southern San Bias Jays are omnivorous. They seemed to forage about equally in palms, herbaceous growth, hedgerows, and jungle patches. During the breeding season they did not usually forage in flocks. There was little or no mixing of birds from different communal groups during the nonbreeding season. At that time of year they typically foraged in flocks and were less vocal than in the breeding season.

We speculate on the way in which each Cissilopha species' characteristic group size, number of breeders per group, and age at first breeding, may be related to one another and to the productivity of the habitat.