The North American population of an endangered seabird, most of which nest on a few small islands in Buzzards Bay, is higher than at any time since 1987, providing scientists with a feeling of optimism following a period of decline in the 2000s that had them worried about the birds’ future.

Yet the roseate tern — a gull-like bird with a black cap, pointed wings, and a sharp beak — still faces threats from predators and climate change that require constant vigilance so the recent gains aren’t lost.

Ninety percent of the population nests on three islands: Bird Island and Ram Island in Buzzards Bay, each of which are home to about 1,100 nesting pairs; and Great Gull Island off the eastern end of Long Island, where 1,800 pairs nest. The remaining 400 pairs nest on a dozen islands scattered from Nova Scotia to New York.

“We don’t know what caused the decline, just as we don’t know what’s causing the increase,” said Carolyn Mostello, a coastal waterbird biologist with the Massachusetts Division of Fisheries and Wildlife, who has monitored the terns in Buzzards Bay for more than 20 years. “That makes it really hard to have confidence that the gains are going to be permanent. It doesn’t allow us to relax anything we’re doing.”

Mostello and a team of eight biologists and students are spending almost every day of the breeding season — May through mid-July — monitoring the roseate terns on Bird and Ram islands, as well as on Penikese Island, another island in Buzzards Bay that has a small nesting population. They count and monitor every nest, assess the growth rate of every chick, band as many of the birds as possible, and conduct a variety of research studies. This year they are evaluating whether the banding process impacts the health and breeding success of the birds.

Gulls, which eat the eggs and chicks, are the terns’ primary predator, so the research team does its best to keep gulls from nesting on the islands and discourage them from getting close to the tern nests. Peregrine falcons are also an occasional concern, since they will eat the adult birds, as are any mammals such as mink, raccoons, or rats that somehow find their way to the breeding islands.

Climate change is a growing concern, according to Mostello. Because the islands are very low-lying — for example, Bird Island’s maximum elevation is just 10 feet — erosion and sea-level rise could reduce nesting habitat, and major storms could flood active nests.

Offshore wind turbines are also an increasing threat, especially with hundreds of turbines proposed for the waters just south of the breeding islands.

“Those are areas that the roseates fly through, so we’re really concerned about those projects,” Mostello said. “Even if each turbine doesn’t kill a lot of birds per year, they’ll be operational for a lot of years, and when you have a rare species that’s long-lived and produces few young per year, it starts to knock down the survival rate and could have an impact on the population. Hundreds of turbines could be a big risk to the terns.”

In an effort to boost the birds’ population, MassWildlife teamed with the Army Corps of Engineers and a number of other partners to restore habitat at Bird Island. By filling in some low-lying areas, planting native vegetation, and increasing the height of the seawall, the project has doubled the amount of potential nesting habitat on the 2-acre island.

“Before the restoration, the birds were very crowded, and that resulted in a lot of agonistic interactions,” Mostello said. “Their territories were small, so neighboring adults were attacking other adults and chicks, resulting in lower productivity. Now they can spread out a bit, they’re less aggressive towards each other, and the substrate is better for them. We have more habitat and it’s better habitat.”

A similar habitat-restoration project is in the planning stages for Ram Island.

Despite the improved habitat and recent population increase, Mostello isn’t ready to claim victory for the birds.

“If you have a population that fluctuates a lot — we went from 2,900 pairs to 4,400 pairs in six years — you would want to wait a while to make sure the population was actually stable before you considered them recovered,” she said. “They could be headed for a downturn. The rate of increase has slowed. It could be that we’re headed for a leveling off and a decline. Only time will tell.”

In the meantime, Mostello and her team will continue to spend almost every day of the breeding season keeping an eye on the roseate terns in Buzzards Bay, knowing that their progress could easily be reversed without a regular human presence.

“If we didn’t show up, we might get away with it for a year, but by the second year you’d have predators that knew they could feed uninhibited on the terns, you’d see declines in productivity, and partial or full abandonment of the colony,” Mostello said. “Having a human presence is non-negotiable.

“While we need to continue to shepherd them through the world, we’ll do it with the hope that someday they’ll be self-sufficient and won’t need this level of effort. We’ve been committed to this species for a long time; we have a huge responsibility here in Massachusetts with 50 percent of the continental population here, so we’re not about to slack off and lose the gains that we’ve made.”

Rhode Island resident and author Todd McLeish runs a wildlife blog.