Here’s one I’ve been putting off for a while. I thought of it when I stepped onto the back deck to catch the ever-ascending March sun last week and gaze into the morning woods’ twig-strewn, graying snow. A couple of squirrels running between dreys in the stacks of fallen firs and brush. Blanketing it all, silence.

It was a kind of eerie, woodland-of-the-damned feeling. I put my aural concentration to work and finally heard a cry in the distance, probably a chickadee. One chickadee. I remembered yesterday being cheered by a blue jay squawk. One squawk. What is the sound of one blue jay squawking? This can’t be right. Where are the birds?

Maybe they are, to a large degree, gone. “Decline of the North American Avifauna,” a study published last fall, reported that since 1970, North America’s bird population has decreased by about 30%. The researchers estimate that nearly 3 billion birds that used to be in the woods, grasslands and shore are gone.

The researchers compiled data from annual bird counts, such as the North American Breeding Bird Survey, the Audubon Christmas Bird Count and the International Shorebird Survey, along with weather radar data that indicate bird migrations, and tracked population figures for 529 bird species, which account for about 90% of North America’s total bird population. They found that grassland birds have declined by 53% since 1970, shorebird populations by 37%. “More than 90% of the total cumulative loss can be attributed to 12 bird families … including sparrows, warblers, blackbirds, and finches,” a preliminary paper from the study states.

The researchers said they began the project thinking they would discover that some bird populations have decreased while others have increased, and they expected the increases would balance out the decreases. They did indeed find that some bird families, such as turkeys and grouse, ducks and geese, raptors, vireos, have gained population. But the gains nowhere near offset the losses. Three billion lost, overall. Three billion birds.

What’s happening to them? Well, the purpose of the study wasn’t to determine causes. Its aim was to find out how many birds there are now compared to how many there used to be. But even though the scientists drew no conclusions about causes, enough is known from many other studies that the main problem is probably habitat loss. As woods, shorefronts, wetlands, grasslands get disturbed or eliminated by building construction, roads, herbicides and — among the most destructive — large-scale industrial agriculture, there are fewer places for birds to live and less food for them to eat. So among the effects are starvation, reproduction problems and untimely death.

Birds are well-known barometers of ecological health. When birds in a certain area are suffering, it usually means the whole area is suffering. So in a way this study is telling us that the whole ecology of North America is suffering.

Of course, the scientists do not say it this way, because they stick to what their evidence and analysis tell them for certain, or as near to certain as you can be with the tools at hand. University of Maine biological science professor Brian McGill in a blog post last fall emphasized in some detail that the study, while probably offering very good estimates for the percentage of bird losses over the past 50 years, at the same time tackled very complicated data requiring numerous assumptions; the figure 3 billion birds may be misleading, for example, because so many of the losses occurred among birds that already are very abundant.

So scientifically speaking, the silence in my spring woods isn’t necessarily connected to overall declines in bird populations. And maybe this February finishing as one of the warmest ever recorded in Maine isn’t necessarily connected to global warming. Maybe the Troy woods have always been silent in early spring.

I don’t think so, though. Rachel Carson warned us about it nearly 60 years ago. We don’t have any time left to put off doing something about it.

Dana Wilde lives in Troy. You can contact him at naturalist1@dwildepress.net. His recent book is “Summer to Fall: Notes and Numina from the Maine Woods,” available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.