Robust but transient thermoregulatory responses to an acute thermal challenge in a free-living altricial songbird.

High temperatures are becoming the new normal as heat waves increase in frequency, intensity, and duration. Experimental evidence shows that acute heat triggers thermoregulatory mechanisms in songbirds, yet the persistence of these thermoregulatory responses remains unclear, particularly among altricial young that may be especially vulnerable. Here, we experimentally tested how a short-term heat challenge influences free-living nestling tree swallows (Tachycineta bicolor) in the wild. We conducted experiments at two key developmental stages: day 6, when nestlings experience their peak of post-natal growth, and day 12, when they approach asymptotic mass. At both developmental stages, nestlings exhibited a robust physiological response to heat, elevating blood heat shock protein (HSP) gene expression. However, HSP gene expression no longer differed from controls within 24 h after heat exposure and remained at control-like levels one week later, suggesting that a single heat challenge does not fundamentally alter this element of heat physiology. We also found that heat exposure elicited clear behavioral responses: nestlings panted more and shifted position within the nest to avoid heat, even at just 6 days post-hatch. However, these putatively beneficial adjustments traded off with begging, suggesting that thermoregulatory responses may constrain food intake at a critical period of growth. Understanding these immediate responses and behavioral trade-offs provides critical insight into how small endotherms may fare under increasingly extreme thermal conditions.