|Male superb fairy-wren (Malurus cyaneus)|
Fairy-wrens are parasitised by cuckoos, so one would expect fairy-wren parents to have evolved ways to distinguish their own offspring from cuckoo chicks in the nest. One way fairy-wren mothers seem to do this, is to teach their chicks a special password before the chicks have hatched. Cuckoo eggs are laid later, so their chicks probably don't have time to learn this special password. At any rate, once they hatch, only the fairy-wren chicks are able to reproduce the password in their begging calls, so they get fed, while the cuckoo chicks are often rejected.
Another endearingly complex aspect of fairy-wren biology, is that in spite of their being socially monogamous, about 2/3 of a single clutch are fathered by males other than a female's social mate. What's more, males can be seen courting neighboring females with flowers, while females don't respond to these cuckolding attempts till just before dawn, when they sneak off for a spot of what biologists so fetchingly call "extra-pair copulation". Not surprisingly, sperm competition in these fairy-wrens is superbly high.
Colombelli-Negrel, D., Hauber, M. E., Robertson, J., Sulloway, F. J., Hoi, H., Griggio, M. & Kleindorfer, S. 2012. Embryonic Learning of Vocal Passwords in Superb Fairy-Wrens Reveals Intruder Cuckoo Nestlings. Current Biology, 22, 2155–2160.
Rowley, I., and E. Russell. 1997. Fairy-Wrens and Grasswrens: Maluridae. Oxford Univ. Press, Oxford.