Inspired by thsi column:
http://www.straightdope.com/columns/read/2287/if-you-handle-baby-birds-will-their-parents-shun-them
If mommy and daddy birds shunned eggs or babies that don’t smell familiar, cowbirds and cuckoos wouldn’t survive. After all, cowbirds and cuckoos lay their eggs in other birds’ nests, then take a powder. The robins or flycatchers who raise the baby cowbirds don’t seem to notice that the new babies don’t smell like theirs.
The reason birds will abandon a nest that has been disturbed is that nests are usually built in places that the bird thinks are inaccessible to predators. By disturbing the nest, you have just demonstrated that their building location was a poor choice. Further, you may have disturbed cover that the birds were counting on to hide the nest. You think you just moved some branches aside so you can see better. The bird thinks, “Oh, crap! Now the snakes and foxes and falcons can see my nest!”