Here's why:
* They are multicellular: Birds are made up of many cells, just like all other animals.
* They are heterotrophic: They cannot produce their own food and must obtain it from other organisms.
* They are eukaryotic: Their cells have a nucleus and other membrane-bound organelles.
* They move: Birds are capable of movement, whether flying, walking, or swimming.
* They reproduce sexually: They reproduce by laying eggs and having offspring that develop from those eggs.
Birds are a fascinating and diverse class within the animal kingdom, with a wide range of adaptations and behaviors.