The characteristics that qualify birds are:
- warm blooded (endothermic)
- vertebrates (they have a backbone and an internal skeleton)
- breathe using lungs (not gills)
- an outer covering of feathers - this feature is not shared with any other vertebrate group
- reproduce by laying eggs
Other features which are characteristics of birds, but not used to set them apart from other vertebrate groups, include:
- All birds have wings, but not all birds fly: generally, there is reduction of skeletal weight
- Birds have beaks or bills, and do not have teeth
- Birds have two legs
- Birds have a cloaca, a cavity into which the intestinal, urinary, and genital canals empty
- Birds have nucleated red blood cells, unlike mammals
- Birds have a chambered heart