Egrets and other birds building nests near ponds that harbor crocodiles or alligators should be suspicious of sticks they see floating in the water. That twig or branch may be bait for a trap set by the reptiles, as new research suggests they may have joined the few species that use tools to lure prey.
According to Valdimir Dinets, a zoologist known for his studies of crocodilian (crocodiles and alligators) behavior, two species of the large reptiles have been observed using twigs and sticks as bird lures.
“At least one of them uses this method predominantly during the nest-building season of its prey,” he writes in a paper published in the journal Ethology Ecology & Evolution. “This is the first known case of a predator not just using objects as lures, but also taking into account the seasonality of prey behavior. It provides a surprising insight into previously unrecognized complexity of archosaurian behavior.”
According to the study, the use of objects as hunting lures is very rare among animals, only seen in captive capuchin monkeys, a few bird species and one insect.
Writing in the study, Dinets states that “it is common for some bird species to preferentially nest in trees growing in ponds with large numbers of crocodiles or alligators, apparently using the crocodilians as protection against tree-climbing nest predators such as snakes, monkeys and raccoons.”
But, he says, the birds have to “pay” for the protection because their chicks can sometimes fall into the water where they are usually devoured by crocodilians.
“But the protection seems to be worth the cost,” Dinets writes. “Almost any crocodile farm or alligator park with appropriate trees will sooner or later become the location of an egret rookery.”
Dinets says he repeatedly saw crocodilians laying in shallow water with small sticks or twigs across their snouts around rookeries.
“The crocodiles remained perfectly still for hours, and if they did move to change position, they did it in such a way that the sticks remained balanced on their snouts,” he writes.
Dinets says the predators get the sticks onto their snouts by submerging under them.
“Then it takes some balancing act to keep the sticks in place,” he said in an email.
Observing alligators at two egret nesting sites for a year, Dinets saw stick displaying mostly during the bird’s breeding season, between March and June and more frequently during nest building, from late March and April.
Dinets noted that the increase in the behavior could be explained by higher amounts of sticks in the water, either because of nest building or trees shedding. He thinks, however that that explanation “seems unlikely.”
“Virtually no freely floating sticks or twigs were seen by the observer at that time, and none are visible in photographs of rookery ponds made at that time,” writes Dinets. “Any available sticks were probably quickly picked up by birds looking for nest material.”
Also, he notes that the most common trees around the waters don’t often shed branches.
Dinets says it’s “unknown” what factors lead to the stick-displaying behavior at particular locations and at certain times of the year.
“The predators might be reacting to the presence of large numbers of wading birds flying low over water, to the sounds made by courting birds, or to some other environmental clue,” he writes.
He also doesn’t know if the behavior is a learned behavior or an evolved instinct.
While Dinets says crocodilians have historically been viewed as “lethargic, stupid and boring,” the new research adds to the complex behavior already known such as signaling, advanced parental care and “highly coordinated group hunting tactics.”
“These discoveries are interesting not just because they show how easy it is to underestimate the intelligence of even relatively familiar animals, but also because crocodilians are a sister taxon of dinosaurs and flying reptiles,” he concludes.