Sources of variation in baseline and stress-induced blood glucose levels in two free-living tropical passerine species

Abstract. Blood glucose plays an essential role in the body's energy metabolism; however, sources of variation in baseline and stress-induced changes in glucose concentration remain poorly understood in tropical passerines. This study assesses intra-annual variation in baseline and stress-induced glucose concentration changes in two common free-living Afrotropical passerines, the village weaver (Ploceus cucullatus) (Müller, 1776) and the black-crowned waxbill (Estrilda nonnula) (Hartlaub, 1883), captured around Dschang, western Cameroon, using intrinsic (sex, body mass, moult and breeding status) and environmental (season, temperature and rainfall) variables as predictors. In black-crowned waxbills, we found that baseline glucose was significantly elevated in the dry season, with the same trend observed in the village weaver. Stress-induced elevations in glucose concentration were observed in the black-crowned waxbills in the dry season, but only when the temperature was fitted as a covariate. Village weaver females showed higher stress-induced changes in glucose compared to males. Body mass and moulting were not predictors of baseline or stress-induced changes in glucose concentrations. This study reveals that some fluctuations in glucose levels may be mediated by seasonal changes in temperature (but not rainfall), sex, and breeding status.