It was an unexpected gift.

We had a few minutes of free time and decided to go for a walk at the Colby College Arboretum in Waterville. As we strolled along we noticed a single bird sitting like a sentinel on the tip-top of a tall aspen at the edge of clearing.

It was clearly a flycatcher, as it sat upright and hovered out to snap at flying insects. We had already heard and seen a variety of flycatchers — eastern kingbirds, eastern phoebes, eastern wood-pewees and great crested flycatchers — so at first tried to fit this bird into the mold of one of these. But it wasn’t just right for any of these common, locally breeding flycatchers.

When it suddenly turned to face us and showed off its stylish gray-brown vest and big-headed gestalt, we realized why: it was an olive-sided flycatcher.

Olive-sided flycatchers are called by some, “boreal pewees,” and with good reason. Not only are they a close relative to the familiar and smaller eastern wood-pewee, but the bulk of their breeding range occurs across the Boreal Forest Region of Canada and Alaska. They do breed in northern part of Maine and other parts of New England and New York and occasionally farther south, but the bird we saw at the Colby Arboretum almost certainly did not breed there.

We, like most birders, don’t see many olive-sided flycatchers every year. They winter in the Andes of northern South America and are amazingly late-returning migrants in the spring. In fact, it’s not uncommon to find northbound migrants still passing through Maine during the first week of June. On the breeding grounds they can be conspicuous because of their loud, whistled song that alcohol-deprived loggers (and now birders) describe as sounding like “Quick-Three-Beers.”

The song can be heard from a surprising distance, but even in the heart of their breeding range, territorial birds tend to be at widely spaced intervals. This is probably because their preferred nesting locations tend to be openings and edges near ponds and beaver flows within spruce and spruce-fir forests. They are also often attracted to forest openings, some of which occur naturally, but also ones that result from logging operations.

There has been some recent research that suggests that olive-sided flycatchers attracted to large clearcut openings may experience lower nesting success because the habitat is not actually as good as it first looks to newly arrived birds. Such an area — one that seems to fit a species’ perception of good habitat but really is not — has been termed an “ecological trap.”

Bobolinks, those grassland-loving, exuberant, black-and-white blackbirds (males) once so common in Maine, face a similar dilemma when they settle to nest in a prime-looking hayfield in the spring only to suffer nest losses when, as is too often the case, the field is mowed at the height of their nesting season.

Olive-sided flycatchers are declining across their range, though it’s not clear what the primary factors are. Along with being very late summer arrivals, the species is also one of the earliest fall migrants. Thus they spend more of their annual life-cycle in their South American wintering range than in their northern breeding range.

During their fall migration, they’re often silent, at most giving out an occasional “pip” call that can be easily overlooked, resulting in an entire population passing through with only a few detected. Check out those flycatchers sitting at the top of high trees on the edges of clearings in the next few weeks and you may be one of the lucky few to see one of these “boreal pewees” on their way to South America.