PHILADELPHIA — One of the world’s rarest and most valuable books is out of the vault and on public view as part of an unusual daily ritual at the nation’s oldest natural history museum.

Every weekday at 3:15 p.m., a white-gloved staff member of the Academy of Natural Sciences lifts the locked protective cover from 19th century naturalist John James Audubon’s influential book, “The Birds of America,” and turns a large linen-backed page to reveal the bird of the day. More than 180 years after Audubon created the life-size illustrations that now link his name with ornithology, their vibrant watercolors and fine details are still remarkable.

“Many times these were framed as artworks and faded from exposure to light,” said curator Robert Peck, who does many of the page turnings. “Ours weren’t exposed to light, so they’re in wonderful condition.”