What ARE birds?

Are birds animals that live on continents?

Birds are not animals that live on continents!

Are kiwi birds?
Are cassowaries birds?
Are moa birds?
Are dodos birds?

Kiwis are birds!
Cassowaries are birds!
It's difficult to tell whether moas were birds, but based on their close relatives, probably.
It's difficult to tell whether dodos were birds, but based on their close relatives, it's highly unlikely.

Are the elapsid snakes birds?
Are sea turtles birds?
Are Komodo dragons birds?
Are the crocodilians birds?
Are Galapagos giant tortoises birds?
Are tuataras birds?

The elapid snakes are birds.
Sea turtles are birds.
Komodo dragons are birds.
Crocodilians are birds.
Giant tortoises are birds.
Tuatara are not birds!