On how birds protect their eyes: division of labour between the upper lid,lower lid and the nictitating membrane

Authors
Citation
E. Curio, On how birds protect their eyes: division of labour between the upper lid,lower lid and the nictitating membrane, J ORNITHOL, 142(3), 2001, pp. 257-272
Citations number
34
Language
TEDESCO
art.tipo
Article
Categorie Soggetti
Animal Sciences
Journal title
JOURNAL FUR ORNITHOLOGIE
ISSN journal
0021-8375 → ACNP
Volume
142
Issue
3
Year of publication
2001
Pages
257 - 272
Database
ISI
SICI code
0021-8375(200107)142:3<257:OHBPTE>2.0.ZU;2-T
Abstract
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 Oscine s; or the upper lid moves down (Psittaciformes, Trochili), or both lids clo se the eye as in Strigiformes and Caprimulgi. Such information is absent fo r most orders, or the handbooks provide wrong or conflicting information. B eside 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 t ypified by their different co-ordination and function. Photographic and obs ervational evidence strongly suggests mechanical protection of the eye as a novel function (where this had not been proposed previously). When an impa ct from any object is imminent from in front of or above the head, the uppe r lid shuts in pigeons, owls and oscines, and with water splashing, the low er lid as well (Cinclus). The most convincing evidence for mechanical prote ction comes from the deployment of the upper lid during the picking up of s piny insect prey as compared to easy-to-swallow berries, when both lids sta y at rest (Gallicolumba). A new function is also attributed to the beating of the nictitating membran e (Membrana nictitans). Traditionally viewed as a cleaning device it also s erves to protect the eye from mechanical impact, and it also can be tuned t o 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 stimul i eliciting the blinking of a lid can, in different species, trigger the be at of the membrane, and can cause it to move tonically (Falco). The membran e beats at a rate difficult to measure since many of its beats coincide wit h the blinking of the upper lid and thus remain hidden (Otus). This coincid ence is difficult to account for by any function discussed so far, as are t he many hidden beats during tonic eye closure with the lids (Passer). The hypothesis according to which the action of the membrane is filtering o ut undesirable retinal stimulation during e.g. rapid head movements is dism issed on four different grounds. The increase of the membrane activity duri ng 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 minimiz ing the exposure of the cornea during foraging along the underside of branc hes. a continual rain of bark particles and debris jeopardizes unimpeded vi sion. Secondarily, one or both lids or the nictitating membrane have taken on the function of optic signals by virtue of contrasting feather colour or color ation. The phasic ('flashing') signal movements involved are directed at th e pair mate (Cinclus, Corvidae, Cepphus), predators (Anas) or at unknown pa rties (Ficedula).