Over time, a study released Wednesday says, egrets, herons, ibises and storks that nest on islands developed a strategy. They nestle on tree limbs near alligators, which chase and sometimes eat nest raiders. For that service, alligators demand a heavy price — some of the birds' offspring. That's right: child sacrifice.

Birds often produce more chicks than they can care for, so one falling out of a nest to be eaten by a gator when the chick would otherwise starve is just nature being cruel. They're not exactly shoved out, but grow weak from neglect, or are accidentally bumped by siblings wrestling for food.

For years, anecdotal evidence suggested that birds engage in this type of behavior all over the world, said the study's lead author, Lucas Nell, who was a student at the University of Florida during the two-year research that ended in 2014. But he looked for something new — whether alligators enjoyed the benefits of this Faustian bargain.

As it turns out, according to the study published in PLOS One, they do. Gators that inhabit tree islands with noisy bird colonies are six pounds heavier than gators that live about a half-mile away. "They're just taking advantage of what they know to be a food source," Nell said. It's less that they know they're protecting the birds, and more that they know food can sometimes drop from on high.

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Herons and other wading birds in the study seemed to know exactly what they were doing. Three years ago, Nell said, a fellow researcher who was not a part of his study tested this theory. She placed fake alligators on an island long before the birds arrived during their migration, and sure enough, most of them hung out on the limbs above the fake alligators. Also telling, no raccoon or possum showed up.

"Our study is the first to demonstrate a mutually beneficial relationship between nesting birds and a crocodilian," Nell said. "Nesting wading birds provide nutrition for alligators that, by their mere presence, create predator-free space for birds."

Data from studies 20 years ago — when researchers marked nests, counted chicks and returned within weeks to see whether they were still there — showed that babies were obviously disappearing. Their lifeless bodies were nowhere to be found. But alligators were everywhere.

Most fall out of nests, Nell said. In years when the swamp's water level is high during breeding season, tree colonies of birds can support most breeding female alligators that will soon have babies of their own. Females near nests tended to have superior body conditioning and a nice layer of fat for child production.

To prove their theory, the researchers, who included another University of Florida scientist, Peter C. Frederick, caught female alligators by noose and by hand, about 40 in all. Some were captured in sloughs within 250 yards of islands with colonies of up to 800 birds. Others were caught more than a half-mile from active colonies or near islands where a colony had not been visited in five years.

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The target areas were the Everglades and Florida Water Conservation area near Miami-Dade and Broward counties, and a wildlife refuge near Palm Beach County. The researchers sampled the blood of the animals to determine what is the equivalence of their body-mass index.

Nell emphasized that the relationship between alligators and wading birds on the tree islands isn't like some sort of cartoon where they strike a deal. "There's not some harmonious relationship," he said. Alligators will waste no time attacking an adult heron or egret, and they're known to slap their tails against trees in an attempt to dislodge even the chicks parents are trying to keep.

Florida alligators are desperate for food. Contrary to popular thought, Nell said, the Everglades is an awful place for them, too. It's the southernmost end of their range, often where water and food sources are low. Alligators in the swamp tend to be skinnier than those in Louisiana and Mississippi, and even those farther north in Florida.

Wading birds contribute to the alligators' diet when they give up a few chicks to protect many. But when raccoons and possums invade colonies, the birds abandon them because the mammals will eat every single egg and chick. Alligators become a necessary evil.

"It's like keeping a murderer in your yard to keep out a cat burglar," Nell said. Except the burglar is also a murderer.