Causes and consequences of female-biased dispersal in a flock-living bird, the Pinyon Jay

Pinyon Jays (Gymnorhinus cyanocephalus) live in permanent flocks of 50-500 individuals on apparently undefended home ranges. We present date collected from 1973 to 1982 on the movement of individuals into and out of a color-banded flock. We banded 681 wanderers from other flocks, 46 of whom immigrated and bred in the study flock. Three hundred and eighty-three juveniles and yearlings born in the study flock disappeared from the natal flock; 22 of these bred in a neighboring flock. One was collected in 1973 as a yearling 600 km from its place of birth. Most dispersal occurred in the fall, but yearling females also dispersed during the breeding seasons. Fall dispersal was primarily by juveniles and included males and females in equal abundance. Females transferred between neighboring flocks more often than males. Many males returned to their natal flocks after months of wandering. As a result, 89% of male yearlings, but only 48% of female yearlings, in the study flock were born there. Annual variability in dispersal by yearlings was correlated with sex ratio in their natal and neighboring flocks; females joined flocks with strongly male-biased sex ratios and females left flocks with female-biased sex ratios. Low status of juveniles and yearlings, especially females, may explain why some dispersal occurred each year regardless of the sex ratio of the flock. Homing ability and the number of relatives in a flock were only weakly related to the percentage of yearlings and juveniles emigrating. Five consequences of dispersal were: (1) flock members may become familiar with resources outside their home range; (2) the sex ratio of yearlings approaches 1:1, despite extreme and variable sex ratio biases among yearlings born in the flock; (3) in some years the yearling cohort is predominantly composed of immigrants and in other years natives predominate; (4) immigrants have fewer relatives in the flock than natives have; and (5) the effective population size ranges from 222 to 370 and includes breeders of a flock and breeders in neighboring flocks.