Researchers at Lund University in Sweden were interested in understanding how birds adapted to cold weather. Birds can’t turn up the heat in their birdhouse, so it is important for them to find physiological ways to stay warm. While they can store body fat and grow more feathers for insulation, metabolism is the primary way endothermic animals create body heat. Mitochondria produce cellular energy (ATP) but can also create heat when the process of making ATP is disrupted. It stands to reason then, that the more this process is disrupted, the more heat mitochondria may produce.
Bird red blood cells contain a nucleus as well as mitochondria. Researchers compared how mitochondria function in red blood cells collected from Coal tits (Periparus ater), Blue tits (Cyanistes caeruleus), and Great tits (Parus major) during autumn as well as winter. These species are related, but differ in body size with Coal tits being the smallest and Great tits the largest. The researchers found that red blood cells isolated from these birds had more mitochondria and higher metabolism during the winter as compared to autumn. This means that bird blood can actually be a source of heat for dealing with cold weather.
A Nord, NB Metcalfe, JL Page, A Huxtable, DJ McCafferty, NJ Dawson. Avian red blood cell mitochondria produce more heat in winter than in autumn. The FASEB Journal. 35(5): e21490, 2021.