Seeing a bird puff up into a little round ball can be endearing, but it can also be a worry. The key is whether it is a brief, relaxed fluff or a constant, listless one.
When puffing up is normal
Birds fluff their feathers for lots of harmless reasons: to trap warm air and keep cosy, while resting, sleeping or feeling relaxed, and to shake their feathers back into place after preening. A bird that puffs up briefly but is otherwise active, bright and eating is usually just fine.
When it's a warning sign
The concern is a bird that stays puffed up for long stretches, especially alongside other signs — sitting low or on the cage floor, sleeping a lot in the daytime, tail bobbing with each breath, or not eating. Because birds fluff up to conserve heat when unwell, a persistently puffed, quiet bird is often genuinely sick.
How to tell the difference
Ask whether the puffing is brief and relaxed or constant and listless. A cold or sleepy bird perks up and smooths down as it warms or wakes; a sick bird stays fluffed, dull and still. With small pet birds especially, hours can matter, so if in doubt it is always safer to contact an avian vet.