What is the most notable characteristic of birds which not found in any other animals?

The most notable characteristic of birds that is not found in any other animals is the presence of feathers. Feathers are complex, lightweight structures that provide birds with insulation, waterproofing, and the ability to fly. They consist of a central shaft with barbs branching off to either side, and they can vary greatly in size, shape, and color depending on the species of bird. Feathers are made up primarily of keratin, a fibrous protein that is also found in human hair and nails. They are incredibly efficient at trapping air, which helps to insulate birds against heat loss and provides buoyancy when they are flying. In addition to their role in flight and insulation, feathers also play a crucial part in bird communication, courtship, and camouflage.