Piloerection

.....as exhibited by one of these cold pigeons. Before the blip censors move in, I should explain that this lad is fluffing his feathers up to trap a little extra air to keep them warm. Piloerection is the stiffening of the hair of the skin or, simply, goose bumps. It’s “pilo-” from the Latin “pilus” meaning “hair”. In the case of birds, piloerection is the reflex of extending their feathers out to keep themselves warm.
So now you know....:-)