The nesting season, in depth

Ten years ago this June, I added a new twist to my birding: I began covering a U.S. Geological Survey Breeding Bird Survey (BBS) route on the Vineyard. Like the better-known Christmas Bird Count, the BBS is a vast, ongoing “citizen science” project. Tapping the talents of thousands of birders across the continent, this project has evolved into the most powerful tool we have for large-scale monitoring of nesting bird populations.

Begun in 1966, the BBS consists of about 3,500 survey routes in the U.S., while the Canadian Wildlife Service coordinates another 1,000 routes in Canada. To make results comparable from route to route and year to year, observers follow a carefully defined procedure. Every survey consists of 50 stops a half-mile apart along a fixed route; at each stop, the observer spends three minutes watching, listening, and recording every bird that is identifiable. Results from around the continent are compiled and made available for analysis by ornithologists. Over time, the BBS data reveal even quite subtle trends in bird populations.

The Vineyard BBS route starts at Lobsterville, with an official start time of 4:37 am, then follows Lighthouse Road, Moshup Trail, State Road, the Menemsha Crossroad, North Road, State Road again, Old County Road before turning east along the Edgartown-West Tisbury Road to the final stop, almost across the road from a monument to “Booming Ben,” the last heath hen. While the last few stops can be dull, the route traverses some splendid bird habitat, and does so at the time of day when the birds are most active and human disturbance is at a minimum. It makes for some pretty lively birding.

I tally roughly 50 species on each annual run, and (even more roughly) about 500 individuals. Of the 500 stops I’ve made over the years, I don’t think any have been totally birdless, and at some, sorting out the chorus of birdsong taxes my abilities. At a lively spot, there might easily be ten species of birds visible or audible, with some of those species represented by as many as four or five individuals. With one eye on the clock, I pin down individual songs, build a mental count for each species, and enter numbers on the data sheet as the sampling period ends.

This year’s start, under cloudy skies at the intersection of Lobsterville and West Basin Roads, was cacophonous: the individual bird songs I could distinguish played against a background blur of more distant notes from the incredibly productive coastal scrub of the Tribal Lands. By the end, about four and a half hours later, I was a bit frazzled but had enjoyed a fine list of birds, including some uncommon Island breeders like wood duck, willow flycatcher, and indigo bunting.

I’ve never encountered a true rarity while doing a BBS, but the survey is intensive enough to detect even our most sparse breeders, and every year the route produces at least one bird I find surprising. A stop on North Road a few years ago, for example, produced the distant song of a veery, a thrush that may be the least numerous of the species that breed regularly on the Vineyard. I know they’re out there, but the odds of one singing within earshot of one of my predetermined stops are miniscule.

On the other hand, the Vineyard BBS route undoubtedly under-counts some breeding birds. Often, this stems from the bird’s habitat preference: relatively little of the route, for example, passes the dense scrub oak that is the preferred habitat of prairie warblers on the Vineyard, and so the odd, ascending buzz of this bird’s song is audible at only a few stops. And wood thrushes and scarlet tanagers, both fairly common in deep up-Island woodland, apparently avoid the edge habitat created by roads and so rarely sing within hearing range of a stop. I heard just two wood thrushes and missed the tanager altogether this year.

In other cases, it’s just the luck of the draw. I can’t explain, for instance, how I managed to tally just one house sparrow on this year’s route, a ludicrous result for this abundant bird.

Yet the Vineyard BBS has proven sensitive enough to promptly detect some meaningful changes over the last decade — for example, the establishment of new species as breeders here. Tufted titmice, which quite famously passed by the Vineyard as their range expanded northward in the 1960s and ’70s, somehow gained a foothold at Seven Gates around 1996. Since then, titmice have colonized the Island thinly but thoroughly; the BBS first detected them about six years ago. Even annual variation shows up in my results: this year, for instance, a number of birders have mentioned that Baltimore orioles seem less common than usual, and my route only produced a single one, versus the half-dozen I’d expect.

Birding under any circumstances is fun, but participating in a “citizen science” project like the BBS adds layers of satisfaction. I enjoy knowing my observations will help biologists track bird numbers and changes in distribution. And my annual survey affords me an interesting, in-depth look at the nesting season, each one of which is unique.