Body temperature and heart rate of free living blackbirds

In a seasonal world, organisms are continuously adjusting physiological
processes relative to local environmental conditions. Due to their limited
heat and fat storage capacities, small animals, such as songbirds, must
rapidly modulate their metabolism in response to weather extremes and
changing seasons to ensure survival. As a consequence of previous
technical limitations, most of our existing knowledge about how animals
respond to changing environmental conditions comes from laboratory studies
or field studies over short temporal scales. Here, we expanded beyond
previous studies by outfitting 71 free-ranging Eurasian blackbirds (Turdus
merula) with novel heart rate and body temperature loggers coupled with
radio transmitters and followed individuals in the wild from fall to
spring. Across seasons, blackbirds thermoconformed at night, i.e., their
body temperature decreased with decreasing ambient temperature, but not so
during daytime. In contrast, during all seasons blackbirds increased their
heart rate when ambient temperatures became colder. However, the
temperature setpoint at which heart rate was increased differed between
seasons and between day and night. In our study, blackbirds showed an
overall seasonal reduction in mean heart rate of 108 beats/min (21%) as
well as a 1.2°C decrease in night time body temperature. Episodes of
hypometabolism during cold periods likely allow the birds to save
energy and, thus, help offset the increased energetic costs during the
winter when also confronted with lower resource availability. Our data
highlight that, similar to larger non-hibernating mammals and birds, small
Passerine birds such as Eurasian blackbirds not only adjust their heart
rate and body temperature on daily timescales, but also exhibit pronounced
seasonal changes in both that are modulated by local environmental
conditions such as temperature.