For Florida’s egrets, the advantage of nesting in trees above alligators is clear: if you have a quarter-tonne of hungry top predator beneath you, lesser predators, especially the tree-climbing sort that eat egrets, will stay away.
What, though, do alligators get from this relationship? Well, it turns out the alligator-egret pact is more Faustian than it appears because every now and then the egrets push a chick out of their nests and the alligators eat it.
Sometimes, if not enough chicks are forthcoming, the alligators slap the trees with their tails until one appears, and scientists studying one of nature’s more unlikely symbiotic relationships have concluded that although both sides benefit, it is a grim trade.
In the everglades, wading birds nest on islands in vast numbers. Biologists have put fake alligators on to some islands and shown that birds are more likely to pick them, presumably because the alligators scare away racoons that can ravage nests.
“In order to understand the ecology of this relationship we needed to know what happens to both partners,” said Lucas Nell, from the University of Florida.
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“Certainly, alligators are not being altruistic in any way. They’re very tender parents, but other than that they do not show a lot of compassion,” he said. “There’s a lot of research showing alligators will move significant distances to get to food sources, and these nesting colonies are very conspicuous, smelly and noisy.”
Crucially, the birds’ hygiene is far from their most unappealing habit: they also eject chicks from their nests if they think they have too many.
At the same time “siblingcide”, where the stronger chicks kill off the weaker, is common. The result, particularly when combined with the observed habit among some alligators of shaking the tree, is a steady rain of juicy chicks.
“It turns out that in prolific years it can be a pretty amazing amount of food, enough to support most of the breeding females over the course of the dry season,” said Mr Nell, whose research is published in the online journal Plos One. “There are tens of thousands of nests, and they provide a stream of nutrients.”
Mr Nell’s research has shown that so significant is the effect of this fluffy manna, that a 2m alligator living beneath a bird colony weighs on average 3kg more than one elsewhere. “It’s less like hiring a bodyguard, more like keeping a psychopathic murderer in your yard to scare burglars,” he said.