Scientists have long known that some bird species choose to nest near “protector” animals—creatures that aggressively chase away nest predators. For instance, tiny European fieldfares sometimes choose to nest near merlins, a falcon that grows territorial during breeding season. But these relationships often appear to be a one-way street.
It turns out that many species of long-legged wading birds in the Everglades have a similar relationship with American alligators. But in this case, the benefits go both ways. Birds nesting above the gators get protection from nest predators and the alligators below snack on chicks that fall out of the trees, according to a recent study in PLOS One.
“We have known for some time that ibises, storks, spoonbills and herons seem to always have alligators underneath their nests,” says Peter Frederick, a wildlife ecologist at the University of Florida and one of the study’s authors, in a press release. “Alligators are serving as nest protectors – keeping raccoons out of the colony, which are otherwise devastating nest predators.”
To determine whether the gators received a tangible benefit from living near birds, the researchers captured, weighed and sampled the blood of 39 female alligators in southern Florida living near islands where large numbers of wading birds recently nested, according to Science Daily. They found that the alligators living near the bird colonies are about six pounds heavier than gators half a mile away, and blood tests show that that they were in better health overall.
Previous research also suggests that the species aren't just meeting by chance. Wading birds seem to actively choose to nest over the alligators despite losing one or two chicks per year, which is within normal chick mortality, the study's lead author Lucas Nell tells The Washington Post. This loss to the jaws below is a small price to pay to keep raccoons and possums away, which can devastate an entire rookery.
That’s not to say the birds and gators are on friendly terms. The giant reptiles will take down any bird that gets too close and actively whack trees with their tails to dislodge nestlings.
“They’re just taking advantage of what they know to be a food source. It's less that they know they’re protecting the birds, and more that they know food can sometimes drop from on high,” Nell told the Washington Post. “It’s like keeping a murderer in your yard to keep out a cat burglar.”