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The language of birds: 2.1 Calls as deceitful mimicry

Vocal appropriation of the sounds of other birds, as in wild marsh warblers in Europe, mockingbirds in North America, Lyre-birds in Australia and robin-chats Cossypha sp. in Africa, is globally limited to perhaps a few hundred species. In Europe there are about 30 mimics out of about 400 nesting species of all kinds. This has been commonly labelled "mimicry" which conforms to the dictionary use of the word as "copying minutely", but not to the biological meaning of the term which implies deceit. Batesian mimicry in butterflies is the visual "copying" (over long periods of evolutionary time) by one (palatable) species of another (distasteful) species' appearance in order to deceive a potential predator.

One of the few fully substantiated cases of this in birds concerns a thick-billed euphonia Euphonia laniirostris that deceived individuals of another species by copying their nest-alarm call. Anxious to defend its nest the bird employed a call to get members of

Jay (Garrulus glandarius) (17K)
European Jay Garrulus glandarius
another species to mob the enemy while the caller itself hid in safety! Eugene Morton, an American ornithologist, approached the nest of the thick-billed euphonia, thus casting himself in the role of potential predator. The adult euphonia, instead of giving its own nest-alarm call, gave that of anotherlocal species, and two individuals of that species were sufficiently provoked to mob Morton. Meantime, the nest owner remained hidden and continued its mimicry! The European jay Garrulus glandarius and the African Natal robin- chat Cossypha natalensis are also well-known to mimic the alarm calls of species other than their own when a potential enemy, (usually the observer), is at the nest site or near fledged but dependent young. It could well be that these parent birds are also inveigling other species into helping them.

A case of a different kind concerns a fork-tailed drongo Dicrurus adsimilis in Africa that learnt to whistle like a local shepherd. Such copying could be true (deceitful) mimicry if its effect were to deceive local birds of prey into avoiding the general area of the drongo believing the shepherd to be there. In other words the drongo, knowing birds of prey probably avoid human beings, gives the impression that there is a shepherd about. This assumes, of course, that the local hawk knew the local shepherd by his whistle. A second case concerns a satin bowerbird Ptilonorhynchus violaceus that during its pre-copulatory display includes, predominantly in its imitative song, the calls of the raven Corvus coronoides and the kookaburra Dacelo novaeguineae, both of which are takers of eggs and young birds. The suggestion is that by deceiving a third party - another species that is a competitor with the bowerbird for, say, food -into believing there are ravens or kookaburras about, the bowerbird is less likely to be troubled. One Australian ornithologist once heard a spotted bowerbird Chlamydera maculata imitate a whistling eagle Haliastur sphenurus and the local hens and chicks ran for cover.

By the same argument it is suggested that a mockingbird, anxious to keep out, say, a food competitor of a different species, will weave blue jay Cyanocitta cristata calls into its song. You may be tempted to ask why doesn't the bowerbird or the mocker simply imitate the competitor exclusively? An "x" on hearing another "x", would surely take the hint. The answer must be that a prohibitively high degree of fidelity would be needed. Another possible interpretation of these two cases is that the bowerbird and the mockingbird may be using imitations of large aggressive birds to keep out other bowerbirds and mockers respectively as an alternative means to holding them at bay with their own territorial song!

In some ways perhaps the most remarkable case of deception concerns a forest falcon of the genus Micrastur (the Slaty-backed M. mirandollei) which in Panama acoustically lures prey within range. The bird apparently hides itself in a suitable position, gives copies of a distress cry, and tasty birds come to investigate. Once one such is within striking distance, it is pounced upon and eaten. This behaviour, carefully observed by a British ornithologist, Neal G. Smith, lends some credence to an African claim, not yet fully accepted as fact, that the African crowned eagle Stephanoaetus coronatus whistles like a monkey to lure it to its death.

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