Water birds in the Everglades may be sacrificing the safety of some of their chicks to alligators for the greater good of gaining protection from raccoons or opossums.
“They’re both acting selfishly and it just happens to work out for both of them,” said Lucas Nell of the birds and alligators. Nell conducted the research for the study published recently in PLOS ONE as part of master’s work at the University of Florida.
Previous studies had shown that birds species like great egrets (Ardea alba), little blue herons (Egretta caerulea), snowy egrets (Egretta thula), white ibis (Eudocimus albus), wood storks (Mycteria americana), and tricolored herons (Egretta tricolor) actually choose to nest in willow trees near American alligators (Alligator mississippiensis) when given the choice.
The reason why is complex. The gators send a strong signal of unsafe habitats that animals like raccoons (Procyon lotor) or opossums shy away from. Since these animals are common nest predators, the birds may gain a net benefit if the alligators keep the raccoons and opossums away.
Alligators may eat the odd bird that falls from the nest, but the sacrifice may not be a huge problem for the birds.
That’s because several water bird species practice brood reduction, laying more eggs than they will be able to raise. Based on yearly food availability, they may neglect some chicks on purpose to give the others a better chance at survival into adulthood.
For the gators it’s a win-win situation. The presence of the bird nests ensures the occasional free meal falls into their vicinity in the form of young chicks. But clumsy chicks aren’t the only benefit. Nell says that fish and other aquatic species are also attracted to the areas due to the bird guano that drops from the nests. The gators, in turn, feed on some of the species that eat the droppings.
“Certainly there’s a lot of guano that drops from these food sources,” Nell said, adding that some aquatic life feeds on that and pushes the droppings up the food chain.
This relationship is important, both for the birds and for the alligators. As part of the study, researchers caught 20 female alligators from areas near bird colonies and 19 from areas without nearby colonies and tested the individuals for fat, body weight and noted a number of other environmental variables. They found that reptiles near bird colonies were around 13 percent fatter and healthier than those that weren’t. This equated, Nell said, to a six-foot alligator weighing around 6 pounds more on average near birds.
“We found that in prolific years for birds, it could actually support the energetics of a large proportion of the breeding population of these alligators,” said Nell, now a research technician at the University of Georgia.
He added that while studies haven’t previously looked at other crocodilian species around the world, similar relationships are likely as species like caimans in South and Central America and Nile crocodiles have been observed in similar relationships with bird species. But for the Everglades species, the relationship could be especially important as the alligators there are less healthy than elsewhere in their range.
“These nesting colonies of birds could be a strong factor in allowing them to persist in these inland marshes.”
|Joshua Rapp Learn is a science writer at The Wildlife Society.