Interior least terns on the comeback

By Thomas Mills

At least a couple of days a week during the interior least tern nesting season biologist Tonya Dunn can be found on the rivers of the Tulsa District in an airboat counting interior least terns. The small birds, often seen fishing along the rivers of Oklahoma, have been listed as an endangered species by the U.S. Fish and Wildlife Service since 1985.

As the lead biologist in the Interior Least Tern Program with the U.S. Army Corps of Engineers Tulsa District, Dunn conducts surveys below the dams that have an impact on nesting terns.

“Conducting surveys includes going out on the river and counting how many adults, nests, fledglings and chicks and whatever else there may be out there,” Dunn said.

The efforts to save the interior least terns appear to be helping as the numbers of birds have been on the rise, or at least holding steady, Dunn said.

The interior least tern is the smallest of terns, and it nests on island sand bars on the Arkansas and Canadian rivers in Oklahoma. They are migratory birds that arrive in Oklahoma mid-May and by the first half of June they are nesting. If there are no floods, and the nests survive, chicks will hatch within 3 weeks.


The greatest threats to the least terns are predators like coyotes and other birds, as well as human disturbances. The terns typically nest on sand bar islands in the rivers, which keeps away most terrestrial predators, and they also don’t normally nest within 400 feet of tall structures or trees to avoid avian predation.

“Whenever there’s a disturbance or a predator the adults will fly up, leave their eggs and, if there are chicks, the chicks will lay flat and freeze,” Dunn said.  The adults, she added, will dive bomb the disturbance.

The eggs are extremely well camouflaged, as are the chicks themselves, which helps hide them from predators, but can cause problems when humans are involved. Humans aren’t as effected as smaller predators by the dive bombing terns, Dunn said, keeping the adults in the air longer, which can cause the eggs and chicks to overheat.

“In addition to that,” Dunn said, “the eggs are so well camouflaged you can actually step on a nest and crush all the eggs and not even know it.”

To conduct a least tern survey the Tulsa District biologists go onto the rivers where the birds nest, usually on an airboat, and count the birds.

“Here in the Tulsa District we have several multi-purpose projects,” Dunn said. These include Kaw, Keystone, Eufaula and Denison Dams, and the terns historically nested below those reservoirs.

“The operations of those dams may impact the least terns,” Dunn said. “We don’t want to jeopardize their continued existence, so we work with the Fish and Wildlife Service and we go out and monitor the population to see how the population is doing and if our operations during the time the least terns are here…[are] impacting the successful rate of their reproduction.”


Besides gathering data on the health of the bird population the biologists also bring back information Tulsa District water managers can use to vary their water releases from the dams. The water releases are part of an Interior Least Tern Plan, said Micah Buchholz, the Upper Arkansas Basin water manager, a plan that juggles the federal requirements of saving the least tern habitat while also keeping the public safe.

“The biologists will tell us how much freeboard we have between where the water is and where the nests are and that gives us an indication of how much play we have in the water releases out of [the dams],” said Buchholz.

According to Buchholz it’s a very complex, controlled system of releasing the right amount of water to mimic natural conditions the birds would experience if there were no dams. There is a certain amount of flood water that can be held back to save the birds, Buchholz said.  “But at some point,” he added, “when those reservoirs get up to a high enough point where we’re concerned, we’ll release the water.”

When Tonya Dunn was a child all she wanted to do was be a biologist and work in conservation. Steering an airboat along the rivers of Oklahoma, checking the status of endangered species means, basically, her dream as a child has come true.

“To go through college, get my degree and then actually work at a place like the Corps of Engineers that protects endangered species through their actions,” said Dunn, “and then to actually be a part of helping endangered species, I mean, it doesn’t get any more personally fulfilling than that.”

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