Having access to water is just as important to birds in winter as it is in summer – even more so since many of their usual sources freeze over. But unfrozen water is also super important for another reason: bathing. It may seem counter intuitive, but the ability to bathe actually keeps birds warmer during freezing temperatures. It all has to do with the structure of the feather. That big vein running down the center of the feather is called the “rachis.” From it sprout light, flexible strands of keratin (the stuff that makes up your hair and fingernails) called “barbs.” And each barb contains a set of tiny hooks called “barbules.” The hooks link together, connecting one barb to the next like Velcro. That’s why feathers regain their shape so easily when you run you hand down their length. But when feathers get dirty, the barbules have trouble connecting, leaving gaps in flight and contour feathers that keep the elements out – and lock warmth in. The result is like having holes in your winter coat. By bathing in cold weather, birds remove the dirt from their feathers so that they can connect correctly and keep them cozy when the temperature plummets.