The effect of winter sex ratio on immune function and condition in a differential migrant.

The dark-eyed junco (Junco h. hyemalis) is a differential migrant in which females migrate farther south in winter than males. Hypotheses to explain this difference in migratory behavior have given limited consideration to physiological differences between the sexes, particularly with respect to immune function. We hypothesized that female migratory behavior could either be directly dependent on climate if migratory distance traveled is a product of interactions between climate and inherent sex differences in physiology, or indirectly dependent on climate owing to interactions between climate and aggressive encounters with males which are known to be behaviorally dominant to females and restrict their access to food. We tested the latter of these two hypotheses by measuring condition in females held in two sex ratio regimes in a common winter environment. Holding density constant in 28 replicate populations, we compared flocks composed of 100% female to mixed-sex flocks composed of 20% female. We assessed condition by measuring immune function via complement activity and response to a foreign antigen, keyhole limpet hemocyanin, as well as by indices of mass, fat, and pectoral muscle. Unexpectedly, females in mixed-sex flocks did not differ in condition from females housed only with females, despite increased aggressive interactions in mixed flocked cages. Combined, our results suggest that overwintering with males has no significant effect on female winter condition. Differential migration in the junco may be mediated by sex differences in ability to withstand harsh northern climate, as has been demonstrated in several mammalian systems, rather than dominance interactions.