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No. 61: Jan-Feb 1989

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Egg Mimicry In Cuckoos

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.)

From Science Frontiers #61, JAN-FEB 1989. 1989-2000 William R. Corliss

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  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
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  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987