Considering the roles of climate change, winter habitat, and immune function in a differential migrant.

摘要

Sex differences in migratory behavior can cause the sexes to settle in different locations giving rise to a distributional pattern known as differential migration. Recent changes in climate have had significant impacts on migration in many species. Little is known, however, about the effect of climate change on winter distributions or winter physiology. I compared current winter demography in the dark-eyed junco (J.h. hyemalis), a songbird in which females migrate longer distances than males, to data collected 30 years ago to assess whether warming has led to detectable changes in sex ratio. Comparison of recent and historic data revealed significant geographic changes in sex ratio that are highly correlated with recent climate warming. Females are apparently making shorter migrations than previously. Additionally, I investigated the potential role of immune function in mediating differential migration by evaluating immune function in wintering male and female juncos in both captive and field settings. I predicted latitudinal variation in immune function with increases in immune responses at lower latitudes. Furthermore, I predicted that variation in immune function would be highly correlated with latitudinal differences in winter climate and that changes in distribution, particularly relaxation of the degree of sexual segregation, could have negative effects on female physiology. Immune function varied significantly with latitude, with more robust responses in birds wintering in the southern as opposed to northern United States. Climate, however, appears to play an incidental role in mediating latitudinal variation in immune function, and further study is merited of geographic differences in other environmental factors which may mediate immune function as well as migratory decisions. Field studies also indicated no significant differences in immune function between the sexes, and this finding was corroborated by a captive study that revealed no effect of male presence on female immune function. The lack of sex differences in immune function in male-biased environments and within latitudes suggests differential migration is not a product of sex differences in immune response and that there may not be consequences for females wintering in the presence of males. Thus, a relaxation in sexual segregation may not be detrimental to this species.