Nictitating membrane

2015, Elizabeth Royte, Vultures Are Revolting. Here’s Why We Need to Save Them., National Geographic (December 2015)https://web.archive.org/web/20151213095110/http://ngm.nationalgeographic.com/2016/01/vultures-text As the wildebeest shrinks, the circle of sated birds lounging in the short grass expands. With bulging crops, the vultures settle their heads atop folded wings and slide their nictitating membranes shut.