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

Body temperature (T-b) affects animal function through its influence on rates of biochemical and biophysical reactions, the molecular structures of proteins and tissues, 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). Here, using thermal radio-frequency identification implants, we quantified the repeatability of T-b, both in the context of a fixed average environment (similar to 21 degrees C) and across ambient temperatures (6-31 degrees C), in a free-living population of tree swallows (Tachycineta bicolor, n=16). By experimentally trimming the ventral plumage of a subset of female swallows (n=8), we also asked whether the repeatability of T-b is influenced by the capacity to dissipate body heat. We found that both female and male tree swallow T-b was repeatable at 21 degrees C (R=0.89-92), but female T-b was less repeatable than male T-b across ambient temperature (R-female =0.10, R-male =0.58), which may be due to differences in parental investment. Trimmed birds had on average lower T-b than control birds (by similar to 0.5 degrees C), but the repeatability of female T-b did not differ as a function of heat dissipation capacity. This suggests that trimmed individuals adjusted their T-b to account for the effects of heat loss on T-b. Our study provides a first critical step toward understanding whether T-b is responsive to natural selection, and for predicting how animal populations will respond to climatic warming.