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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Why acts the gaping of the cuckoo Cuculus canorus nestling as a supernormal releaser?
Auteurs :	Josef H. Reichholf, Auteur
Type de document :	Article
Dans :	Ornithologischer Anzeiger (43(2) August 2004)
Article en page(s) :	205-210
Résumé :	It is widely accepted that mouth colouration of the hatched Cuckoo nestling acts as a "supernormal releaser" not only onto the foster parents but sometimes even to other songbirds passing by, despite the fact that gaping not simply presents an enlarged version of the normal stimulus which the feeding adults could await. In fact, the Cuckoo's mouth colouration clearly differs in most cases from that of the host species' nestlings. Nevertheless the signal is essential for function and evolution of the Cuckoo's brood parasitism. The argument presented here considers the brilliant and very intensive red mouth colouration of the young Cuckoo as a highly reliable signal indicating an excellent health status. Presenting such a colouration has been decisive quite likely for starting the feeding of altricial nestlings after hatching. The amount of food a nestling receives until fledging, is influenced substantially by that signal probably since the beginning of passerine birds' evolution. Because it indicates the viability of the offspring. In precocial birds the performance of the young provides directly visible clues to the parents, but for altricial ones a reliable signal is necessary for preventing investment into offsprings which are not likely to survive. The greater the parental investment, the better the signal has to be - and the red (or yellow) mouth colouration provides such a "honest signal" because it is a mirror of the amount of "free" carotenoids which have not been used up already for fighting internal parasites or diseases. Since most, if not all small passerines "understand" the meaning of this signal, the Cuckoo still has a broad spectrum of possible host species, and a closer matching with the actual pattern and colouration of the gaping mouth of the foster parents' nestlings is not necessary. The young Cuckoo's gaping mouth size and brightness, therefore, can - and does so actually - act as a supernormal releaser.

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