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

Body temperature (Tb) 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 Tb 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 Tb is important for predicting how animal populations
will respond to climatic warming, given that endotherms maintain Tb that
are close to the upper lethal threshold.  Here, we quantified the
repeatability of Tb, both in the context of a fixed average
environment (~21°C) environment and across ambient temperatures, in a
free-living population of tree swallows (Tachycineta bicolor). by
experimentally trimming the ventral plumage of a subset of female
swallows, we also asked whether the repeatability of Tb is
influenced by the capacity to dissipate body heat. We found that the
Tb of both female and male tree swallows was highly repeatable
at 21°C, and with increasing ambient temperatures. The
repeatability of female Tb 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 Tb is
a repeatable trait in a free-living bird, and that avian Tb could
therefore be responsive to natural selection.