Study suggests tiny birds' blood gets warmer in the winter One of the coal tits that was part of the study Andreas Nord Andreas Nord In order to stay warm on frigid winter days, small birds may do more than just fluff up their feathers. According to new research, they're actually able to make their blood run hotter, creating a sort of central heating system. In a recent study conducted by Sweden's Lund University and Scotland's University of Glasgow, scientists took blood samples from great tits, coal tits and blue tits, both in early autumn and in late winter. The researchers isolated red blood cells from those samples, then observed the mitochondria activity within those cells.