The Alligator in the Basement: Breeding Bird Colonies Benefit Alligator Health in the Florida Everglades

Even the most dedicated reptile-lover wouldn’t want an alligator hanging around the school playground.

But for waterbirds in the Florida Everglades, having an alligator near their nesting colonies keeps out other unwelcome visitors. And the arrangement is good for the gators, too. Scientists have discovered that alligators living near nesting bird colonies are healthier than those without access to this food resource, according to new research published today in PLOS One.

Gators & Birds: Not Such an Odd Couple

It’s a scene familiar to anyone who has puttered around South Florida waterways by kayak, canoe, or powerboat. Sawgrass stretches across the wide horizon, while alligators lounge beneath cypress and willow trees festooned with perching egrets, ibis, heron, and anhinga.

“Crocodilians are often found underneath these nesting birds, and this has been a common natural history observation for quite some time,” says Lucas Nell,  who conducted this research as part of a master’s in wildlife ecology at The University of Florida.

But what may not be obvious at first glance is that this idyllic tableau is anything but coincidental — alligators and wading birds benefit and depend upon one another.

Previous research by Brittany Burtner found that wading birds do indeed prefer to nest on tree islands adjacent to alligators — something she discovered by putting out fake, plastic alligators and birds throughout the Everglades. Logic follows that there had to be some benefit to nesting near such an obvious predator, and the suspicion is that the alligators were keeping out even more unwelcome neighbors — raccoons and Virginia opossums.

“If raccoons get into a nesting colony it’s just devastating,” says Nell. He explains that thousands of birds will abandon their nests if a raccoon infiltrates the colony because the birds have no real defense against them. “A great egret will peck away at a raccoon and the raccoon will just kind of ignore it and eat the chicks,” he says, leaving nothing but the litter of little bird legs behind.

Burtner found that tree islands surrounded by high water, and therefore gators, have fewer numbers of raccoons or possums, likely because the alligators dissuade — or eat — the mammalian predators. That’s good news for the birds, but what benefit do the gators get from this relationship?

Making Housecalls in the Glades

Nell set out to quantify exactly how beneficial it is to an alligator to have a territory with a breeding bird colony. Focusing on two sites in the Everglades, Loxahatchee National Wildlife Refuge and Water Conservation Area 3A, he used a combination of Google Earth and airboat survey data to identify tree islands with and without colonies of breeding birds.

Then he and his research partners hit the field to capture gators. “You go out at night and you shine your spotlight across the water and you look for their little eyes,” says Nell. “Then you sidle up next them in the airboat… and slip a noose over their neck and cinch it down.” Next, they use a combination of a second noose and tape to secure the gator’s mouth before hauling them aboard the airboat.

In June of 2013 and 2014, they captured 39 female alligators. (Because female alligators have small smaller home ranges than males, their health and body condition would be more reflective of prey opportunities from the bird colonies).

Once the gator was on board, Nell and his team took blood samples and measurements to calculate a morphometric index, which Nell likens to an alligator Body Mass Index, or BMI. A factor of the animal’s weight and snout-to-vent measurement, this index is a good measure of the gator’s health.

Nell’s results revealed that alligators with nesting colonies in their territories measured 13 percent higher on that index compared to alligators with no nesting colonies nearby. “For about a 6-foot gator, they will on average weigh about 6 pounds more if they’re near a colony,” he says.

They didn’t find any major differences in the alligators’ bloodwork, but Nell says that’s not entirely surprising, because even non-colony-adjacent alligators did have access to food resources and weren’t starving.

An Uneasy Alliance

However much alligators and nesting bird colonies benefit from one another, Nell cautions that it’s not an idyllic relationship. “Nesting above an alligator is less like having a bodyguard and more like having a psychopath in your yard to keep out catburglars,” he says.

Nell says that there are reports of alligators slapping against tree trunks to knock wobbly-legged chicks into the water below, and his colleague once observed a small gator actually scampering up the side of a tree to snatch an ibis chick out of a low-lying nest.

Nesting above an alligator is less like having a bodyguard, and more like having a psychopath in your yard to keep out catburglars.

Lucas Nell

Alligators benefit wading birds in other ways, besides this perverse protector-turned-predator relationship.

“Alligators act as both apex predators and ecosystem engineers,” says Nell. As they wallow and wade through the waterways of the Everglades, alligators create holes and small ponds that retain water even in the drier months and years when water levels drop precipitously. “Those ponds create refuges for fish and aquatic invertebrates when it’s extremely dry,” says Nell, “and sometimes those are the only organisms of those types to survive in extremely dry years because that’s the only water around.”

Nell hopes that this research will encourage other ecologists to tease apart similar mutualistic relationships elsewhere in the tropics and subtropics, where other species of wading birds and crocodilians live in close proximity.

It may be an uneasy alliance, but it works: Alligators get the occasional airborne snack, and birds get protection from predators. That is, as long as they stay out of jaws-reach.