Climate change is altering the frequency and intensity of environmental extremes, and the diurnal rates of environmental change. The daily reaction norms of wild animals show spatial and temporal plasticity to allow appropriate physiological responses to predictable environmental challenges, but these responses have rarely been quantified in wild birds. We addressed this by determining whether physiological stress indices (corticosterone [CORT], glucose [GLU], and circulating heat shock proteins [Hsp]) vary with ambient temperature (Ta) or time of day across a 60 latitudinal gradient (320–380 S). We sampled house sparrows, Passer domesticus, at three locations with varying daily rates of Ta increases in 2015 and in 2019–2020 to determine whether physiological responses to capture and handling stress change diurnally. As predicted, plasma CORT and GLU increased during stress. Baseline CORT and GLU levels did not change during the day, but the amplitude of plasma CORT increase (delta CORT) in response to capture decreased as Ta increased, as was the case for GLU, in birds with initially high baseline CORT. Neither baseline nor stress-related plasma CORT or GLU differed consistently across sampling locations. Heat-shock cognate 70 (Hsc70) and heat-shock protein 90α (Hsp90) showed clear temporal dynamics across the day. Thus, the reaction norms of heat shock proteins are temporally plastic during the day and in response to daily Ta changes, as are the reaction norms of CORT and GLU in response to acute stress. However, the study provides little evidence for such plasticity in reaction norms as a function of average local thermal conditions.
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