Body temperature is a repeatable trait in a free-ranging passerine bird

<p><span>Body temperature (T_b) affects animal function through its influence on rates of biochemical and biophysical reactions, the molecular structures of proteins and tissues, metabolic rate, and ultimately, organismal performance. Despite its importance in driving physiological processes, there are few data on how much variation in T_b exists within populations of organisms, and whether this variation consistently differs among individuals over time (i.e., repeatability of a trait). Understanding the repeatability of T_b is important for predicting how animal populations will respond to climatic warming, given that endotherms maintain T_b that are close to the upper lethal threshold. </span> Here, we quantified the repeatability of <span>T_b, both in the context of a fixed average environment (~21</span>°C) environment and across ambient temperatures, in a free-living population of tree swallows (<em>Tachycineta bicolor</em>). by experimentally trimming the ventral plumage of a subset of female swallows, we also asked whether the repeatability of T<span>_b is influenced by the capacity to dissipate body heat. </span>We found that the T_b of both female and male tree swallows was highly repeatable at <span>21</span>°C, and with increasing ambient temperatures. The repeatability of female T_b did not differ as a function of heat dissipation capacity, thereby supporting the hypothesis that individual performance is limited by the capacity to dissipate body heat. To our knowledge, our study is the first to provide evidence that T_b is a repeatable trait in a free-living bird, and that avian T_b could therefore be responsive to natural selection.</p>