I was walking down Market Street the other day, obviously cold, and saw this small group of pigeons hanging out by the Cable Car Turnaround at California Street.  They looked a bit fat, I thought, like they were completely fluffed up.  I knelt down very close to them and snapped some photos.  To my surprise they didn’t even budge a tiny bit!

Apparently, it’s piloerection. That’s when they fluff their 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 because even birds do get cold.