One-fourth of all the illustrated covers National Geographic magazine has published in its 126-year-long history have featured animals.

The newly published National Geographic: The Covers shows that birds lead in the cover sweepstakes, with apes and gorillas in second place. Snakes and mollusks lag far behind, and in April 2014, a hedgehog made a single, solitary appearance on the cover for a story on wild pets.

Why do birds lead the pack? "It's not so much about bird species," points out Kathy Moran, the magazine's senior editor for natural history. "They're usually stand-ins to represent environmental issues." For example, the brown pelican coated in oil on the cover of October 2010, which illustrates a story on the Deepwater Horizon blowout in the Gulf of Mexico.

The photographers who make these cover images are specialists. Natural history photography is a game of wait, then wait some more. After all, you can't make an appointment with a rhinoceros, pay a cheetah to pose, or, as underwater photographer David Doubilet says, corner a fish like paparazzi in a nightclub.

The payoff? Natural history photographers are often the front-line reporters on the environment.