In the swampy Florida Everglades, egrets and herons build nests that hang precariously close to alligators swimming beneath, ready to swallow any weak chicks that fall into the murky waters.

This game of survival -- while dangerous for the birds -- brings benefits to both, according to researchers who published their work this week in the journal PLOS ONE.

By nesting near alligators, the birds take advantage of a certain degree of protection. Alligators tend to deter furry animals like possums or raccoons that might steal birds' eggs or hatchlings, scientists say.

In exchange, the alligators are pretty much guaranteed a steady meal, as birds are known to pare back their broods by sacrificing the weakest, pushing them out of the nest when times are hard and food is scarce.

The result is a fleet of well-fed gators. And they make no promises about which birds they will or won't eat.

"We were surprised by the magnitude of the nutritional benefits that alligators were getting," lead author Lucas Nell, a researcher at the University of Georgia, told AFP on Friday.

"A six-foot alligator near a (bird) colony will, on average, weigh about six pounds more than one not near a colony."

- Trading food for safety? -

Nell and colleagues followed up on a previous study by a separate team that tested to see if birds really did prefer to nest near alligators, by putting fake alligators near tree islands.

They found that raccoons and opossums were scarce in areas were the fake alligators lurked, and birds nested in much higher numbers, suggesting that "birds perceive that alligator presence mean better nesting habitat," Nell said.

His team measured the fatness of alligators that lived near birds' nests and compared them to those that did not.

"Alligators near colonies were much fatter and healthier," he said.

So is this a case of birds buying the alligators' protection with a weak chick to eat now and then?

Or have birds decided that furry predators are a greater threat than alligators, and are simply choosing the lesser of two evils?

"As far as we know, they are both just acting in their own self interests. We have no evidence that birds are actively 'paying' alligators or that alligators are making a concerted effort to protect nests," said Nell.

"Birds just want safe nesting areas, and alligators eat anything that hits the water."

Nesting near alligators does not guarantee survival for the parent birds. Plenty of evidence shows that the creeping reptiles will leap out of the water to nab a bird.

Alligators have also "shown some interesting problem-solving behavior when food is involved, like slapping tree trunks to knock chicks out," added Nell.

"There is even some evidence that they might use branches to lure in birds when birds are building nests."

Hence, this reptile researcher said he has a clear opinion on which creature is smarter.

"I'd give the prize to the alligators, but admittedly I'm pretty biased."