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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	On how birds protect their eyes: division of labour between the upper lid, lower lid and the nictitating membrane
Auteurs :	E. Curio, Auteur
Type de document :	Article
Dans :	Journal of Ornithology (142(3) JUL 2001)
Article en page(s) :	257-272
Langues:	Anglais
Résumé :	Birds close their eyes during sleep in various taxon-specific ways. Either the lower lid moves up as in the majority of species including the Anseres, Accipitres. Falconiformes, Galli, Charadrioidea, Columbiformes, and Oscines; or the upper lid moves down (Psittaciformes, Trochili), or both lids close the eye as in Strigiformes and Caprimulgi. Such information is absent for most orders, or the handbooks provide wrong or conflicting information. Beside the tonic, sleep-related eye closure, birds move one or both lids in a phasic, usually swift mode when awake. These frequent lid movements are typified by their different co-ordination and function. Photographic and observational evidence strongly suggests mechanical protection of the eye as a novel function (where this had not been proposed previously). When an impact from any object is imminent from in front of or above the head, the upper lid shuts in pigeons, owls and oscines, and with water splashing, the lower lid as well (Cinclus). The most convincing evidence for mechanical protection comes from the deployment of the upper lid during the picking up of spiny insect prey as compared to easy-to-swallow berries, when both lids stay at rest (Gallicolumba). A new function is also attributed to the beating of the nictitating membrane (Membrana nictitans). Traditionally viewed as a cleaning device it also serves to protect the eye from mechanical impact, and it also can be tuned to the side from where danger is threatening, though by and large there is a dearth of information from avian taxa. The non-visually elicited action of the membrane seems always to be bilateral (Falco, Harpia). The very stimuli eliciting the blinking of a lid can, in different species, trigger the beat of the membrane, and can cause it to move tonically (Falco). The membrane beats at a rate difficult to measure since many of its beats coincide with the blinking of the upper lid and thus remain hidden (Otus). This coincidence is difficult to account for by any function discussed so far, as are the many hidden beats during tonic eye closure with the lids (Passer). The hypothesis according to which the action of the membrane is filtering out undesirable retinal stimulation during e.g. rapid head movements is dismissed on four different grounds. The increase of the membrane activity during predator alarm (Ficedula) is functionally unaccounted for. In a fashion unique among birds, the Blue Nuthatch (Sitta azurea) protects its eyes by contracting the naked skin surrounding the eye, thereby minimizing the exposure of the cornea during foraging along the underside of branches. a continual rain of bark particles and debris jeopardizes unimpeded vision. Secondarily, one or both lids or the nictitating membrane have taken on the function of optic signals by virtue of contrasting feather colour or coloration. The phasic ('flashing') signal movements involved are directed at the pair mate (Cinclus, Corvidae, Cepphus), predators (Anas) or at unknown parties (Ficedula).

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