It could be for one of three reasons.
First of all, birds puff their feathers to trap warm air inside them, to keep warm. This usually happens when they are about to sleep. When a bird sleeps, you should ideally cover the cage with a towel or bird cage blanket to block out the light and the draught.
Secondly, alert birds who are not sleepy or cold will puff up their feathers slightly when they are relaxed. Similar to sleeping or cold birds, they are trapping air to keep their temperature steady. The bird is happy. You can usually tell when a bird is relaxed as it will also usually stand on one foot, relaxing the other one.
Thirdly, birds will puff up their feathers and appear twice their normal size if they are angry or aggressive. This will happen if you disturb the bird (ie - tapping the cage or waking it from sleep) or if the bird is threatened by the appearance of another animal, such as a cat. The bird will be standing on both feet and watching its surroundings constantly or giving the source of annoyance a "dead-on stare".
finch ?
it either means hes angery or hes getting rid of un needed heat. or even showing off.
shakes its tail feathers.. and puffs up. then poops on its prey.
approximately 4 months
Your finch may be stressed from the environment/ size of the cage or whatever bird is with her may be plucking them out!
The phrase to ruffle your tail feathers means to annoy someone and get the angry, when a ducks becomes frighted or is bothered it makes itself seem bigger by ruffling its feathers so it puffs up.
coco puffs
mountain bluebird
If it is the feathers around the head it is called flaring(this means he wants to attack). If the whole chicken puffs up or drags one or both wing on the ground it is called strutting(this means he wants to mate, but be warned it can also mean he wants to attack).
It means to take two puffs every 4-6 hours.
Ducks that have puffs of feathers on their heads are various types of domestic ducks called "crested" ducks.
Possibly about to lay eggs. Finch can be touchy about eggs however and if not given proper nesting materials and shelter will most of the time not sit the eggs. If she is fluffing up her feathers could also be a cold or upper respiratory issue.