All birds had a common ancestor species. Anything that's a descendant of that ancestor species is a bird. The actual ancestor hasn't been found, but it can be

All birds had a common ancestor species. Anything that's a descendant of that ancestor species is a bird. The actual ancestor hasn't been found, but it can be determined by genome studies, and by comparing known details (usually from the skeleton, because that is the only thing available from most fossils). Essentially they collect a lot of traits of many species. If you assume that more closely related species have more traits in common a computer program can then figure out the most likely evolutionary tree. In that tree, anything in the bird branch below the "bird ancestor" node is a bird.