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NEW YORK — It was one of the more curious reasons for a flight delay: scores of turtles — diamondback terrapins, to be precise — slowly marching across the tarmac at John F. Kennedy International Airport.

The incident, in 2009, naturally drew headlines. While there had always been turtles in the Jamaica Bay Wildlife Refuge, just south of JFK, their occasional presence on the airport’s grounds did not normally cause a stir.

But on that July day five years ago, Russell Burke, chairman of the biology department at Hofstra University, said: “Something made a huge number of turtles come up to Runway 4L.”

Two years later, it happened a second time. And Thursday, a large number of turtles appeared on Runway 4L again, despite recent steps aimed at keeping them away.

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For Burke, who has long studied the terrapins that live in the wildlife refuge, the reptiles’ repeated forays onto the tarmac are the subject of serious study. Shortly after the first invasion, the Port Authority of New York and New Jersey enlisted him to help it come to a better understanding of the turtles and their mysterious ways. As part of that effort, he works closely with the Port Authority’s chief wildlife biologist, Laura Francoeur, who described the turtle takeovers as among the more vexing challenges confronting her unit.

“The more information we have about them can help us know why they’re coming here and how we can manage them,” Francoeur said. “This has been a really odd, out-there issue that we’ve had to deal with.”

To help determine what was drawing the turtles onto the runway, Francoeur hired Jeff Kolodzinski, a senior wildlife biologist. Working together starting in the fall and through this spring, the pair helped lay miles of black plastic tubing, the kind used to prevent soil erosion, along the airport’s razor-wire border. The barrier stopped many turtles from climbing up from the nearby shore.

Whenever a turtle does shimmy past the plastic barrier, members of Francoeur’s team are dispatched to fetch it. The terrapins are often seen by pilots, for whom even a small creature on the tarmac is a potential hazard on par with a stray chunk of pavement, dropped bolt or shred of tire. (The most terrapins struck by planes in a single year was six, Francoeur said.)

Once they have the turtle in hand, the biologists probe under its shell with their fingers to feel whether it is full of marble-size, peach-color eggs. If so, they release it over a fence onto a sandy beach where it can nest. If not, the turtle gets a ride to airport headquarters and temporary shelter in a Coleman cooler.

Then, working in an air-conditioned conference room overlooking a section of runway, Francoeur and her team lay the turtles on a mahogany meeting table surrounded by swivel chairs. In plastic bowls, the biologists mix water with powdered alginate, the substance dentists use to make molds of human mouths. The paste is purple, and smells and tastes like cherry candy. It is globbed onto the backs of the turtles until it hardens into a cast. Then the cast is removed and the turtle is returned to the wild.

“You get to know the personalities of different turtles,” said Melissa Zostant, a graduate student of Burke’s and a summer intern with the wildlife biologist team, as she spooned paste onto the back of a squirming terrapin one recent afternoon.

The ridged, green-and-yellow backs of a terrapin have rings, similar to those found within the trunks of trees, and biologists use these marks to estimate an animal’s age. Studying the casts, Burke noticed something about the turtles storming the airport. Many were young, between 7 and 9 years old. This is the age terrapins reach sexual maturity — and when they return to the beaches where they were born in order to nest.

Around Jamaica Bay, the growth of the turtle population has traditionally been kept in check by raccoons, which kill about 95 percent of each year’s crop of newborn terrapins, Burke said. They raid the nests for eggs; they also eat the hatchlings. This, coupled with the ages of the turtles at Kennedy, led Burke to a hypothesis.

“I’m guessing that seven to nine years ago, something happened to the raccoons at JFK,” he said. “Because a bunch of eggs that were laid those years survived, and then they started hitting the runway when they were able to reproduce.”

As it turns out, many raccoons on the wildlife refuge died in 2008 as a result of a distemper outbreak, Burke said. Since then, the turtles have flourished.

Now there are signs that the turtle population at Kennedy, at least, is leveling off. In June 2012, Francoeur’s team counted 800 terrapins. A year later, there were 400. This year, the biologists tallied about 300.

Still, the turtles make occasional visits, such as the one Thursday, when a high tide washed 86 terrapins over the plastic barrier at the marshy edge where runway 4L meets Jamaica Bay. Because that area is so low to the water, the barrier often isn’t high enough to keep the turtles out.

A few turtles made it onto the tarmac, but planes were able to use a different runway, said Ron Marsico, a spokesman for the Port Authority. And though there were some flights delays at JFK on Thursday, Marsico said a more traditional culprit was to blame: the weather.

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