What is the single most important feature used to identify an animal as a bird?

The single most important feature used to identify an animal as a bird is the presence of feathers. Feathers are highly specialized structures found only in birds and play a crucial role in their survival, serving various functions such as insulation, waterproofing, flight, and display. Other notable characteristics associated with birds include the presence of a beak, wings, and laying of amniotic eggs. However, feathers remain the unique and definitive feature that sets birds apart from other animal groups.