MOORE HAVEN, Fla. — In Florida's Everglades, few species are more closely tied to the habitat's health than an endangered bird, the snail kite. The Everglade snail kite is a raptor, similar to a hawk, that eats just one thing: snails.

Over the last century, as much of the Everglades was drained, the bird's population declined precipitously. But the kite has bounced back recently thanks to an exotic snail. It's a rare case of an invasive species having a positive impact.

Robert Fletcher, a University of Florida professor who directs a snail kite monitoring program, says the invasive species was first spotted in 2004. Within a few years, it had expanded through much of the Everglades. "And it was around that time," he says, "that we started to see snail kite number increase."

Few people pay closer attention to the snail kite than Tyler Beck. He manages Florida's the endangered bird's population for Florida's Fish and Wildlife Conservation Commission. On the western edge of Lake Okeechobee, Tyler Beck uses an airboat to motor through marshes looking for kites. He sees one hovering over a clump of trees and cuts the boat's engine. "Hear that call?" he asks. "That's only when they're irritated."

Overhead, the alarmed kite makes a rapid clicking call as it hovers and swoops over the airboat. University of Florida researcher Brian Jeffrey wades through thigh-deep water toward the area he thinks the nest might be. Jeffrey directs a field team that monitors Florida's snail kite population. He finds the nest, but it's 20 feet up, too high to count the eggs or see if any have hatched. Other members of his team will be back soon with a ladder to check on the nest.

Jeffrey has three field teams that cover thousands of square miles counting and tracking Florida's snail kites. The kites—and the field teams--range from Everglades National Park on the southern tip of Florida all the way up to Paynes Prairie Preserve State Park, near Gainesville. He says, "We have a lot of ground to cover."

Snail kites aren't flashy. Males are a slate gray, females a splotchy brown. They get their name from their ability to seemingly float in the air. They were one of the last bird species discovered in the U.S. because of where they live, often hidden in the Everglades.

Beck says the species is uniquely adapted to subsist almost entirely on a resource usually abundant in the freshwater marshes: apple snails. "They have these really long talons that hook around the shell and get a good grip on it to lift it out of the water and carry it away," he says. And, they have a long, hooked bill they use to pry the snails from their shells.

Over the past century, as much of their habitat was drained and water stopped flowing through parts of the Everglades, the snail kite population plummeted. It was one of the first birds put on the endangered species list in the 1960's. Droughts contributed to the snail kite's decline and by 2007, there were fewer than 800 remaining. "Right shortly after that though, this invasive snail came in, and just started flourishing, getting into every wetland, having these big population booms," Beck says. "Since then, the snail kite population has been slowly rebounding."

By last year's count, there are now more than 3,000 snail kites. And, although it's early in the season, field teams are finding lots of nests with eggs and chicks.

Beck eases the airboat up next to a willow tree where he's spotted a nest. Standing in the boat, this one is at eye level. He says, "We've got two little nestling snail kites. These are probably about ten days old. The parents, you can hear them over us, they're upset that we're at their nest." Beck and Jeffrey mark the location, water levels, height of the nest and then motor away. The parents soon return, carrying snails.

No one's sure about how the exotic snails were introduced into the Everglades. They're related to Florida's apple snails and are commonly used in home aquariums. The invader, the Island apple snail, is found in similar habitat in South America and is larger than its Florida cousin. It lays eggs in the thousands in pink clusters visible on the stalks of many of the marsh plants.

Efforts to restore Florida's Everglades have helped the snail kite, bringing back native vegetation and restoring the flow of water to once-parched marshes. It's been in the works for more than 30 years with a cost of more than $20 billion. But progress is incremental and hard to measure. In the meantime, scientists say the invasive snail may have saved the snail kite.

But University of Florida scientist Robert Fletcher is concerned about the potential impact the species will have on the Everglades over the long-term. He says, "What we should be thinking about is how do we restore native snails to get those benefits rather than relying on this non-native species that can have detrimental impacts on the ecosystem."

The South American snails may already be taking a toll on some native marsh plants, a sign trouble may be looming. In the meantime, they've helped pull Florida's endangered snail kite back from the threat of extinction.

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