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He thinks he is a girl and he spits eggs from his mouth. He'd rather be called "birdetta."
This explanation is not satisfactory. If there's
a big hole that eggs are coming out of, xe doesn't just "think" xe's a girl.
I mean, unless they're implying that it's really just a mouth and xe swallows a whole [dukar]bunch of eggs every time whenever xe's going to show up on camera so xe can spit them out.
But the more likely explanation is that the Birdo's we've seen are all physically female and it's always "that" "time" of the "month" (that's why they're fighting you) and the biologist writing the SMUSA manual had never seen Birdos before (and called them Ostros) and was just confused and didn't know what the hell was going on (again, they called them Ostros, which doesn't even make sense, how do they look like ostriches?).
It is some weird anatomy going on there, but I mean we've got
jellyfish, so it's not that big a leap.