Ecological Correlates of Group Size in a Communally Breeding Jay

Abstract

The authors 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. The authors 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, the 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 bottom, the authors estimated the linear density of the population in May at 1.7 flocks/km or 16.7 jays/km.