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Least Terns Nesting on Sanibel Causeway

June 21, 2024
least terns on sanibel causeway

Least terns have returned to the Sanibel Causeway to nest for a second year in a row.

Last year, these state-threatened seabirds were found nesting on Causeway Island B for the first time in 30 years. This year, nesting activity has been observed on both Causeway Island A and Island B.

“Least terns are colony nesters, meaning they nest in groups. This requires large open areas including beaches, dry mudflats, and gravel rooftops,” said SCCF Shorebird Intern Riona Lahey. “We’ve observed about 50 individuals on the causeway, across two different sites, with a handful of confirmed nests.”

The birds were seen during the first week of major rain this season in mid-June. After coordinating with the Florida Fish and Wildlife Conservation Commission (FWC) and Florida Department of Transportation (FDOT), the SCCF shorebird team successfully posted the area during a rainstorm, providing a safe area for the least terns. The area was posted before eggs were laid, preventing any disturbance to actively nesting birds. 

three people stand in rain
From left: SCCF Shorebird Biologist Audrey Albrecht, Shorebird Technician Aaron White, Shorebird Intern Riona Lahey

“Posted areas are important for beach-nesting birds like least terns to prevent harm from human disturbance, and in this case, harm from nesting in an active construction site,” Lahey said, adding that monitors are currently on site during all construction activities to ensure no harm is done to the colonies. 

Historically, least terns and black skimmers (Rynchops niger) regularly attempted nesting on the Causeway Islands after it was built in the 1960s. Unfortunately, vehicle traffic and summer storms left most nests unsuccessful.

About Least Terns

The least tern (Sternula antillarum) is the smallest tern species in the world, weighing in at just about 1.5 ounces. Along with their small size, adult least terns in breeding plumage can be easily identified by their bright yellow bill and legs, a black crown that extends over their eyes to form a white triangle on their forehead, and elegant, narrow wings.

Their nests are known as “scrapes,” which are small nest bowls dug out in the sand where they lay their eggs. The incubation period for the eggs is about three weeks. 

“Least tern chicks are known as altricial, which means chicks stay with their parents, relying on them for food and protection from the weather and predators. However, they’re not like most altricial chicks because they are downy at hatching and can move around freely, which is more of a precocial trait,” Lahey said. “The chicks stay with their parents for around five weeks until they develop flight feathers and are capable of foraging on their own.”

Least terns typically arrive on Sanibel in early April and depart by August. At the end of the breeding season they head to tropical waters, with some traveling as far south as Brazil. 

As with all beach-nesting birds, it’s important to respect posted areas and observe all birds from a good distance. Flushing birds from their nest may leave their eggs vulnerable to predators and damage from the hot sun.

“Last year, the Sanibel Causeway nests unfortunately failed due to predation and over washing due to tides and low nest locations,” Lahey said. “We hope that this season will be a successful one for these least terns.” 

SCCF’s shorebird team works with partners and volunteers to monitor beach-nesting birds on North Captiva, Captiva, Sanibel, Bunche Beach, and the Sanibel Causeway.

Learn more about our shorebird program.


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