Anthropogenic climate change is dramatically altering conditions across the globe, but there is still much to learn about the physiological mechanisms that facilitate thermal tolerance, particularly in endotherms. Animals may respond to heat by regulating heat shock proteins (HSP), which combat stress-induced cellular damage. Thermal challenges may also alter corticosterone (CORT) secretion, thereby mobilizing resources during metabolically challenging conditions. We explored these mechanisms by which developing birds may acclimate to a naturalistic thermal challenge. Specifically, we exposed free-living tree swallow (Tachycineta bicolor) chicks to an experimentally mimicked 6-day heat wave using disposable air-activated shipping warmers, which elevated nest temperatures by at least 2°C during the period of rapid postnatal development (6-12 days post-hatch). To assess physiological acclimation to heat, we quantified HSP gene expression in the blood across various time points during heat exposure, including 4 hours, 3 days, and 6 days after the onset of heat. We measured baseline and stress-induced CORT secretion at the end of the treatment period. Experimental heating did not affect nestling growth or parental visitation, but initial analyses suggest that nestling behavior was affected. Here we describe both initial and delayed effects of heat on HSP gene regulation and the degree to which HSP expression is correlated with CORT signaling. Heat may be particularly relevant in this system because tree swallows are currently undergoing a unique southward range shift and are breeding in warmer climates. Furthermore, this experiment informs our understanding of how birds respond to climate change and lays the foundation for future work exploring the evolution of thermal resilience.