No. 61: Jan-Feb 1989
In Britain, cuckoos mainly parasitize five species of smaller birds. They do this by laying their eggs in the hosts' nests. After hatching, the young cuckoos grow much faster than the young of the host species. Soon the cuckoo is able to eject the host's young from the nest and get all the food brought by the parents. Actually, the cuckoo is so aggressive in this business of parasitization that, when it finds a host nest with eggs so far along in incubation that parasitization is impractical, it destroys the whole nest. This usually forces the host birds to lay fresh eggs, giving the cuckoo a chance to parasitize the nest.
The most remarkable thing about cuckoo parasitism is the birds' ability to match the eggs of the host species in size, spottedness, background color, and darkness. The eggs of all five species commonly parasitized in Britain are much smaller than a bird the size of the cuckoo would normally lay; but the cuckoo still manages to lay eggs of just the right size. When the bona fide eggs of the five parasitized species are placed side-by-side with the mimics layed by the cuckoos, the matches are uncanny - except in the case of the dunnock, which the cuckoo doesn't try to mimic at all. The question, of course, is how the cuckoos do it.
American cuckoos rarely parasitize the nests of other birds; but the American cowbird is notorious in this regard, although it does not indulge in egg mimicry. On other continents, cuckoos, honeyguides, finches, a weaverbird, and a duck have learned how to slough off parental duties.
(Brooke, M. de L., and Davies, N.B.; "Egg Mimicry by Cuckoos Cuculus canorus in Relation to Discrimination by Hosts," Nature, 335:630, 1988. Also: Harvey, Paul H., and Partridge, Linda; "Of Cuckoo Clocks and Cowbirds," Nature, 335:586, 1988.)