Thank you to the commenters on my previous post-- I now have enough information to put together a theory that makes me happy.
The idea that city pigeons have varied patterns because they've cross-bred with fancy pigeons didn't feel right-- surely fancy pigeons are too rare to make that much difference.
However, ritaxis contributed the idea of a brindle gene which contributes irregular coloring.
If I assume that the brindle gene was picked up from fancy pigeons and it helps with camouflage in the cluttered city environment, the way city pigeons look makes sense. It's not a matter of fancy pigeon genes in general-- some features, like curly feathers, just aren't practical for a wild bird.
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