When you walk into the campground area north of the parking lot at Mass Audubon’s Wellfleet Bay Sanctuary, you’ll likely hear the twittery trill of birdsong emanating from the pines. Look into the canopy, and you’ll see the source: small, long-tailed warblers in the treetops — pine warblers.

The adults’ bodies are mostly gray with a yellow head and breast, as though dipped in mustard. Some of the juveniles have this goldenrod color in patches around the breast and flanks.

Pine warblers may be the most common breeding bird on the Outer Cape, according to sanctuary banders James Junda and Valérie Bourdeau. In the summer, these birds dominate the expanses of pine forests here, then transition to life in the oak forests and fields in the fall. Despite their abundance, there is little research on their ecology.

When the two ornithologists arrived at the sanctuary in 2014 — Junda was hired to take on running the banding station here seasonally and the couple return to their home in Montreal each winter — they were struck by how charismatic and easy to observe these birds were. Starting in 2017, they began studying the warbler population at the campground.

Now Wellfleet Bay’s pine warblers may be the best-studied population of these birds in the world. Junda and Bourdeau’s research has helped reveal aspects of the birds’ behavior that were unknown to science before they started looking.

Pine warblers are wide-ranging birds, found from the Canadian Maritimes south to Texas and the Bahamas. Throughout their range, they breed in the canopies of pine-dominated forests, which puts them out of reach of observers. But here, our nutrient-poor soils and windy conditions keep the pines short, which makes observing these birds easy.

Junda and Bourdeau do their research on a volunteer basis — Bourdeau says that, given the ease of observing the warblers here, studying them felt like a duty they could not ignore.

One of the first things they figured out was how many pine warblers there are here. They outlined a 40-acre study area and mapped each warbler’s territory, discovering 17 pine warbler territories in their grid — that means 17 territorial males, plus their female mates and offspring. This was more territories than for all other bird species in the plot combined.

In their study plot, Bourdeau and Junda also uncovered other key statistics, for instance that pine warbler territories are approximately 1,300 square feet in size. They eat by gleaning insects from bark while perched rather than grabbing insects on the wing. They stick to feeding in pines in the breeding season.

But Bourdeau and Junda wanted to know these birds as individuals, and that would require making each bird uniquely identifiable.

They would find a place where a territorial pine warbler was singing and erect a large, nearly invisible mist net beneath it. They then coaxed the pine warblers down from the canopy by setting up two 3D-printed decoy pine warblers, rigged with string that they could pull to make the decoys appear more alive.

To attract a warbler, they would play a recording of its song while shaking the decoys. This made the pine warblers irate. They came down to investigate who was encroaching on their territory. Once a bird descended, Junda and Bourdeau jumped out of the bushes, startling the bird into the net, where they extracted it and attached a silver band and three unique color bands to its legs.

The elaborate technique may have looked silly, but once they had each bird individually identified, they began to learn more about them. There seems to be a territorial hierarchy in this group of pine warblers. They found that the older birds live in the middle of the patch of pine forest, where the only birds they have to deal with are their neighbors. The younger birds, meanwhile, tend to have territories closer to the forest’s edge, where they are constantly forced to defend it. Junda said that one territory may have as many as four males occupying it, all singing, squabbling, and vying for mates.

The two scientists also began to understand the family lives of these birds and learning how males attract females. In the early spring, the female follows the male around as he makes display flights to impress her. Later in the year, the roles reverse. Once nest building begins and the female is more fertile, the male starts following her around to mate with her. After the eggs are laid, the male brings food to the female as she sits on the nest, warming their brood.

The tagging operation has also revealed how long pine warblers can live. In the fall of 2017, Bourdeau and Junda banded a young male. The next year, he returned and set up a territory, and he’s been coming back ever since. He sports three dark blue bands and a silver metal band, giving him the name DDDS.

As of this year, DDDS is one of the two oldest known pine warblers ever recorded. Junda and Bourdeau said he is likely in the sanctuary now, raising this year’s young with his mate DDKS, who sports two dark blue, a black, and a silver band.

Many of these birds have nicknames: DDDS, Sergio, Christmas. The scientists know when they arrive, know which birds are their mates, and where they spend time. The birds become individuals in a way very few birds are to humans. If DDDS were to fail to return next year, as two other sanctuary regulars did this year, Junda and Bourdeau would surely grieve his absence.

Observing these pine warblers so closely, the scientists have become attached to some of them.

“People often overlook the little birds in their back yards,” said Junda. But the birds in our back yards are interesting. Seeing that is just a matter of taking the time to understand them, Junda said. “The more you study them,” he said, “the more fascinating they become.”