WADA, H.; BREUNER, C.W.; WINGFIELD, J.C.; MOORE, I.T.; University of Texas, Austin; University of Texas, Austin; University of Washington; Virginia Polytechnic Institute and State University: Stress responses in equatorial sparrows: comparing tropical and temperate Zonotrichia

Comparisons between sister species are useful ways to test the role of environment as a selection pressure shaping the adrenocortical response to stress. Previously, Zonotrichia populations breeding at mid to high latitudes were used for such comparisons. In this study, we extend the comparison to Zonotrichia capensis, the only Zonotrichia species that breeds in the tropics (at the equator). Factors such as unpredictability in food and weather during the breeding season may affect how extensively populations regulate glucocorticoid secretion and availability. We examined the seasonal and sex differences in stress response in equatorial population of Z. capensis and compared the results to northern Zonotrichia, especially to Z. leucophrys. Blood samples were collected at 0 and 30 minutes after capture and corticosteroid binding globulin (CBG) and total corticosterone (CORT) levels were measured to calculate free levels of CORT. In addition, we investigated the possible role of CBG as a sex steroid binding globulin as suggested for temperate passerines. Similar to northern congeners, Z. capensis show seasonal modulation in total CORT and CBG capacity with higher levels during breeding season compared to molt. However, unlike many of temperate zone passerines, there are no sex differences in CORT secretion or CBG capacity. Furthermore, all of the seasonal differences in total CORT diminish when the free levels are calculated. We will discuss how the differences in breeding and non-breeding environment may explain the results we see in this study.