I think bird houses are an interesting idea, but they can easily tip the balance. Crafting them from just wood and nails will cause people to just put up LOTS of these so they have lots of eggs for basically nothing (no investment after the initial build). There should be some kind of balancing factor to offset a possible early-game exploit.
Oh, and although I believe I know what you meant by "there aren't birds in the game" (you probably mean small songbirds), a nit-pick: I seem to recall at least two: wandering chickens, and the truly, truly annoying vulture zombie