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CronoDAS comments on The Featherless Biped - Less Wrong
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To go on a bit of a tangent...
Upon seeing this, I decided to try to come up with my best guess as to what the necessary features of a bird are. So I came up with the birdiest non-bird I could think of, thinking qualitatively: why is a bat not a bird? I realized that bats don't lay eggs, but platypuses, a mammal, do; since I couldn't think of a mammal that both lays eggs and flies, but I could think of a mammal that does each, I postulated a hypothetical mammal that does lay eggs and fly, and compared it to a hypothetical bird with fine, hair-like feathers, and pondered what would distinguish these; out of somewhere, the idea came to me that birds' lungs are different somehow. So that was one of my distinguishing features: birds have a certain type of lung.
I also considered the birdiest non-birds I could think of, thinking taxonomically; these were the reptiles. It was easier to come up with a distinguishing feature here: birds are warm-blooded.
So, my thought left me with this best definition: "A bird is a warm-blooded organism with a certain type of lung." Looking up on Wikipedia what type of lung birds have, I was able to refine this into this: "A bird is a warm-blooded organism with circulatory lungs." How close did I come?
Taxonomists have a pretty ugly job, having to fit messy real-world objects into categories and having those categories be sharply defined.
For example, do you want to put mushrooms and other fungi into the category "plant"? It seems like a plant, has a cell wall like a plant, is mostly immobile like a plant, but it doesn't perform photosynthesis!
I only said "taxonomically" instead of "phylogenically" because I couldn't think of the latter word. I would imagine a phylogenist's job to be a bit easier: fungi are more closely related to animals than to green plants; therefore, unless animals are plants, fungi are not plants. I'd say it's pretty darn convenient for the phylogenist that, at least in eukaryotes, everything can be organized into neat, sharply defined categories.