While gnat larvae develop in the boobies' droppings, adults sip moist mucus from the birds' eyelids. They're happy to drink around your eyes, too. And in your ears, up your nose, or on any patch of sweaty skin—by the hundreds, tickling without letup. The green-and-turquoise lizards, Uta palmeri, found only on San Pedro Mártir, are almost as bold. Show a bit of red on your sock, and a dozen emerge from rocks to run up your leg and bite the cloth. Red happens to be the color of a key Uta food: fruit from cardon cactuses, which grow in candelabra forests fertilized by the bird lime. The lizards also dash right in among the birds to snatch fish scraps and gnats.

There are 115 kinds of land-dwelling reptiles on the gulf's islands. Forty-eight of them are unique to this region. For example, the Midriff island called San Esteban has produced a rattlesnake, whipsnake, spiny iguana, and Gila monster-size chuckwalla, all found only within its 16 square miles.

Ana Luisa Figueroa of the Mexican resource agency CONANP (Comisión Nacional de Áreas Naturales Protegidas) urged me to add this: By all means, go see the wonders of the gulf's isles. But do it with a guide or, better yet, from a boat circling the shores. The birds nest so close together that a single hiker can put thousands to flight. Before that blizzard settles down, every exposed egg or young chick will have been eaten by gulls or ravens.

Many of the gulf islands have native Peromyscus mice. Many host the endemic Mexican fishing bat. But hardly any hold larger predatory native mammals. After seeing me off on my snorkel along San Pedro Mártir's shore, Araceli Samaniego, a biologist with the Mexican nonprofit Grupo de Ecología y Conservación de Islas, part of a North American island conservation network, scrambled up steep cliffs where she noticed a dwindling colony of red-billed tropic-birds nesting. She returned fuming, carrying crab shells and infant bird bones, all gnawed. "Rats," she grumbled. "Everywhere!" House mice—Mus musculus—and cats, too, often jump ship to take up island life.

At each island we visit, Samaniego sets out a series of baited live traps. A petite, soft-spoken woman of 28 with an easy laugh, she talks to the wild mice she catches, calling them corazón—sweetheart—and stroking their little feet before letting them go. If the traps hold rats instead, she carefully weighs and measures them. Then she kills them. She doesn’t like it, but she likes the idea of rats gobbling nestlings and rare endemic life-forms even less. Two-thirds of all known extinctions worldwide since the year 1600 have taken place on islands, Samaniego reminds me, and alien species introduced by people are a major cause.