Term for puffed up feathers

ruffle TFD

3. To erect (the feathers). Used of birds.

According to an article titled "How do birds keep warm?" on the Tough Little Birds website, the term for feather fluffing or puffing is ptiloerection:

Feathers: There is a reason why we fill our best coats with goose down. Feathers are fantastic insulation. Downy feathers trap tiny pockets of air next to the bird, allowing the bird to warm those pockets of air and hold that warm air around itself, preventing cold air from touching its skin. The more air trapped, the warmer the bird. Birds fluff up (the technical term for fluffing up is “ptiloerection”) in the cold to trap as much air in their feathers as possible.

Merriam-Webster Online, however, suggests that the correct spelling of the term is piloerection:

piloerection noun (1930) erection or bristling of hairs due to the involuntary contraction of small muscles at the base of hair follicles that occurs as a reflexive response of the sympathetic nervous system especially to cold, shock, or fright

What this definition describes is, of course, the condition familiarly known as goose bumps, goose flesh, chicken skin, etc.

The prefix pilo- derives from Latin pilus (hair). Nevertheless, the connection to birds is strong—and "Do Birds Get Goose Bumps?" on the Ornithology: The Science of Birds website asserts that fluffing is indeed a feathery form of piloerection:

So the papillae function to form feathers, but at their base are small muscles which contract to move the papillae and thus the feathers. If you kept a parakeet at some time in your life, you are familiar with their fluffing up their feathers. They might do it in response to a perceived threat such as a dog or cat in the house, another parakeet, or just a general disturbance. The bird will also fluff up when it is ill, to avoid losing body heat. And that’s what you see in the wild most– birds sitting on a branch in the winter, appearing to be twice their normal size. They piloerect their feathers to produce air spaces between them for insulation.