Alex Hyde / Golden eagle (Aquila chrysaetos) sequence showing protective nictitating membrane over eye. Captive bird, UK.

Anup Shah / Ostrich yawning, eye shut, beak open {Struthio camelus}, Masai Mara Game Reserve, Kenya

Mark Carwardine / Southern boobook owl / Morepork / Ruru (Ninox novaeseelandiae) showing nictitating membranes, Kauri Park, North Island, New Zealand, endemic

Paul Williams / Jabiru stork (Jabiru mycteria) showing nictitating membrane to protect eyes whilst fishing, Pantanal, Pocone, Brazil

Thomas Marent / Red-eyed Tree Frog (Agalychnis callidryas) with half closed eyelid, Tortuguero National Park, Costa Rica

Philippe Clement / Common Blackbird (Turdus merula) female feeding juvenile whilst using the nictitating membrane for protection of the eye. Belgium, June.

Jeff Rotman / Eye of Tiger shark (Galeocerdo cuvier), nictitating membrane closing to protect eye, Great Barrier Reef, Australia, Coral Sea.

Jeff Rotman / Eye of Whitetip Reef Shark (Triaenodon obesus), nictitating membrane closing to protect eye, Cocos Island, Costa Rica, Pacific Ocean.

LYNN M. STONE / King vulture (Sarcoramphus papa) showing nictitating membrane, captive

Graham Eaton / White tailed sea eagle (Haliaeetus albicilla) portrait, captive bred bird, showing nictitating membrane, UK, September

Angelo Gandolfi / Eagle owl (Bubo bubo) portrait with translucent nictitating membrane in mid-blink, captive

Michael Hutchinson / Dipper (Cinclus cinclus), yawning / gaping with nictitating membrane over eye, Derbyshire, UK

Doug Perrine / Tiger shark (Galeocerdo cuvier) lunging for bait, showing nictitating membrane covering eye to protect it, North Shore, Oahu, Hawaii, USA, Central Pacific Ocean

Doug Perrine / Tiger shark (Galeocerdo cuvier) lunging for bait, showing nictitating membrane covering eye to protect it, North Shore, Oahu, Hawaii, USA, Central Pacific Ocean

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas) eye covered with nictating membrane

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas), the golden webbed skin is a nictitating membrane, part of the eye lid and enables the frog to see without giving up its camouflage

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas) eye covered with nictating membrane

Christian Ziegler / Red-eyed Tree Frog (Agalychnis callidryas) eye with retracting nictitating membrane

Jabruson / Emu (Dromaius novaehollandiae) showing nictitating membrane response, UK, captive

Tony Heald / Giant eagle owl {Bubo lacteus} showing pink eyelids, Laikipia, Kenya

Heidi & Hans-Juergen Koch / Red-eyed Tree Frog (Agalychnis callidryas) male with nictitating membrane over eyes, clings onto female in amplexus, stimulating her to lay eggs, native to the tropical rainforests of Central America

Heidi & Hans-Juergen Koch / Red-eyed Tree Frog (Agalychnis callidryas) close-up portrait showing tympanum or eardrum, and nictitating membrane which protects the eyes from danger but does not completely restrict vision, native t...

Heidi & Hans-Juergen Koch / Red-eyed Tree Frog (Agalychnis callidryas) close-up portrait showing nictitating membrane which protects the eyes from danger but does not completely restrict vision, native to tropical rainforests of...

Colin Seddon / Portrait of Great horned owl {Bubo virginianus} showing one nictitating membrane, UK, captive

Grant McDowell / Common raven {Corvus corax} soiled from mobbing thrushes. Note nictitating membrane. Not available for ringtone/wallpaper use.