Ecological Correlates of Group Size in a Communally Breeding Jay

Abstract

We studied local variation in size of social units in the Gray-breasted (or Mexican) Jay (Aphelocoma ultramarina) throughout its elevation range in the Chiricahua Mountains, Arizona [USA]. At its lower limit (1,463 m), the ecological distribution of the species ended where oaks became rare, even though other large trees were present. The upper limit of distribution (2,103 m) coincided with the beginning of the area occupied by Steller''s Jays (Cyanocitta stelleri), and not with a decrease in the number of species of Quercus or the availability of mast. Size among social units varied locally at all elevations. Unit size was not correlated with elevation. We studied yearly variation in a small number of units over 15 years. Mean size of these groups varied from 6.7 to 17.5 individuals. Although group size was highly variable in space and time, our findings reaffirm reports from the Santa Rita Mountains that group size in the Arizona population of this species tends to be unusually large compared to that of other communally breeding birds. The number of juveniles per flock in August was positively correlated with group size in May. Along the canyon botton, we estimated the linear density of the population in May at 1.7 flocks/km or 16.7 jays/km.