Least terns have returned to Buck Island Reef National Monument to nest/Craig O'Neal file via Wikipedia. CC BY 2.0

A year after they failed to show up, a small contingent of least terns has settled down to nest at Buck Island Reef National Monument in the Caribbean, prompting park staff to erect fences and hang out signs warning beach goers to watch where they step and to control their pets.

Buck Island is one of 15 known nesting sites on St. Croix for the least tern, Sterna antillarum, a locally protected migratory sea bird, the Park Service notes. 

The terns lay small speckled, sand-colored eggs in shallow depressions on the beach. Both adults take turns sitting on the eggs, foraging for food, and protecting and incubating the eggs. The terns are easily disturbed by people walking nearby and will leave the eggs and fly toward the invader. Every time a tern flies off the nest during the day, the eggs or chicks are exposed to excessive heat from direct sunlight and to possible predation.

According to the U.S. Fish and Wildlife Service, least terns are the smallest tern in the Virgin Islands. You can identify them by their black cap with a v-shaped white forehead, black-tipped yellow bill, and orange legs and feet during breeding season, the agency says. Non-breeding adults have brown bills and less distinctive white foreheads. The birds nest on beaches and salt flats; occasionally on dirt and gravel beds and industrial sites. They breed from April to July. 

Least tern chicks at St. Croix can be hard to spot/USFWS, Claudia Lombard file

At Buck Island, about 20 adult terns have settled on West Beach to raise their broods. A section of the beach where the terns are has been fenced off from the rest of the beach to keep the birds from being disturbed during their nesting and fledging periods.

"Over the years, the terns have had marginal success because our visitors are not adhering to the closure. The NPS has allowed multiple use activities to continue alongside the tern nesting, however if the least tern nesting continues to fail, the NPS superintendent will be forced to close the beach area to all use for a period of time," park staff note. "The least tern nesting area on Buck Island Reef National Monument is unique to the Virgin Islands. It is the only area where the birds have no threat from vehicles, dogs, and other non-native predators. The only danger to the terns nesting success seems to be from human disturbance and dogs illegally on the beach in the park."

The nesting and fledging season is expected to run into July, and the access restrictions will remain in place until the end.

Least tern eggs also can be hard to pick out/USFWS, Claudia Lombard file