The same can be said about birds. You can say that for an animal to be considered a bird it: a) has to have feathers, b) has to have warm blood, c) cannot have

The same can be said about birds. You can say that for an animal to be considered a bird it: a) has to have feathers, b) has to have warm blood, c) cannot have claws on its wings, d) cannot have teeth. But than you find one animals that we can all agree is a bird that has claws on its wings while still a chick (will it grow up to become a bird?). So you use a new definition. Bird are evolutionary descendant some reptiles had contained feathers and warm blood. This will (mostly) hold out until we discover some extinct groups of reptiles that had feathers and warm blood. Note though how vague the latter definition is. It has to be. Because we cannot derive at a solid definition of a bird that encapsulates all animals that we can consent on calling a bird.