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.

Pigeons