Long-legged wading birds that nest above resident American alligators (Alligator mississippiensis) for protection from mammalian nest predators may also provide a source of food for the alligators living in the Everglades, Florida, says a team of researchers led by University of Florida scientist Lucas Nell.

For many bird species nest predation is the greatest threat to reproductive success.

Long-legged wading birds, like herons, egrets, ibises, storks, and spoonbills may choose nesting sites above resident American alligators, likely to take advantage of protection from nest predators such as raccoons and opossums.

“In mixed-species wading bird nesting colonies in the southeastern United States, medium-sized, arboreal, semiaquatic mammals such as North American raccoons (Procyon lotor) and Virginia opossums (Didelphis virginiana) present the greatest nest predation threat, and these birds have no evolved defenses against such nest predators,” Nell and co-authors said.

While the benefits to the bird species of having alligators nearby are well-documented, few studies have explored the effect on the protector, the alligator.

Specifically, some previous research has shown that wading bird nesting colonies could provide substantial food for alligators in the form of dropped chicks.

In a new study, Nell and his colleagues compared the body condition of 39 alligators living with and without long-legged wading bird nesting colonies in similar habitat in the Everglades, Florida.

To measure body condition, the scientists took blood samples and measured the mass and length of the alligators.

“We captured thirty-nine female alligators (20 colony, 19 non-colony), ranging from 7.5 to 46.0 kg total mass and 146.6 to 239.1 cm total length. Because female alligators in the Everglades can reproduce at 1.5 m total length, we considered all individuals potentially reproductively active and hereafter refer to them as adults,” they explained.

“The bird nesting colonies at which we captured alligators were primarily comprised of great egrets (Ardea alba), with smaller numbers of little blue herons (Egretta caerulea), tricolored herons (E. tricolor), snowy egrets (E. thula), and anhingas (Anhinga anhinga).”

The team found that the body condition of alligators found near wading bird nesting colonies was higher than those in similar habitat without active colonies, independent of a range of environmental variables.

The mean body condition based on size and shape for colony-associated alligators captured for the study ranked as the 63rd percentile, while that for non-colony females ranked as the 17th.

“There appears to be a beneficial interaction between the alligators and nesting birds, where birds nest above alligators for protection, but chicks that fall from the nest may also provide a food source for the alligators,” the scientists said.

“Further studies are needed to analyze a wide variety of alligators living in different habitats, but since nesting birds and crocodilians co-occur in many tropical and subtropical wetlands, the results highlight a potentially widespread beneficial interaction between two ecologically important species groups.”

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

“Crocodilians and nesting birds co-occur throughout the tropics, so these may be globally important ecological associations.”

The findings were published online March 2 in the journal PLoS ONE.

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Nell L.A. et al. 2016. Presence of Breeding Birds Improves Body Condition for a Crocodilian Nest Protector. PLoS ONE 11 (3): e0149572; doi: 10.1371/journal.pone.0149572