As the Earth continues to dramatically warm, experiments on the mechanisms facilitating thermal tolerance can offer important insights into the functional effects of climate change. Previous work, largely studying ectotherms, has established that heat induces various stress responses, including the upregulation of heat shock proteins (HSPs) that counteract cellular damage. However, tissue-level variation in HSP responses remains unclear, leaving uncertainty as to which tissues, and by extension which traits, are more or less buffered from heat. To test the hypothesis that tissues vary in the degree of protection against heat, we experimentally simulated a naturalistic 7-day heat wave inside the nestboxes of wild tree swallows (Tachycineta bicolor). Using air-activated warmers, we elevated nest temperatures by at least 2°C, starting when chicks were 6 days old. In heated and control nests, we collected morphological data and measured HSP gene expression in the blood every third day. At the end of the experiment, we collected additional tissues, including the brain, and used qPCR to measure HSP gene expression. We compared tissues to begin exploring which physiological functions may be most affected by heat. We also assessed the degree to which HSP gene expression is correlated with other fitness-related traits, such as mass and wing length. Exploring mechanisms of thermal tolerance in tree swallows is particularly relevant because these birds are currently undergoing a unique southward range shift, breeding in warmer climates, suggesting they may be adapted to heat. More broadly, this experiment informs our understanding of how endotherms cope with the rising temperatures of climate change.