Photo by Pixabay.com • Birds are disappearing. Some populations have seen a dangerous decline. Loggerhead shrikes are declining across the continent, and the reasons are complicated but can ultimately be traced to human activity.

Imagine the sky growing dark and, looking up, you notice that the cause is not approaching storm clouds but a passage of birds — millions of individual birds, their wings darkening the skies as they pass overhead.

The early American naturalist and painter John James Audubon once described the passage of enormous flocks of passenger pigeons that blotted out the light “as by an eclipse” and described the noise of the multitude of wings “like thunder.” His observation of these flocks took place in 1813. A century later, the world’s last passenger pigeon, a species that had ranked as one of the continent’s most numerous birds, died in 1914 at the Cincinnati Zoo.

Photo by Bryan Stevens • Legislation like the Endangered Species Act can and does save birds like the Bald Eagle from possible extinction.

The passenger pigeon was the avian equivalent of the American bison, albeit with a more tragic outcome. Bison, also commonly called buffalo, still survive. As with the bison, we’ve had avian rescue success stories — whooping cranes, Kirtland’s warblers, bald eagles — with efforts to bring some birds back from the brink of extinction. At the same time, we’ve lost others, including the ivory-billed woodpecker, the eskimo curlew and Bachman’s warbler. Now a new study indicates that our birds may be under assault as never before.

Photo by Bryan Stevens • A young Northern cardinal visits a feeder.

The journal Science dropped a bombshell article recently about declining bird numbers in North America. The article’s claim that nearly 3 billion — that’s billion with a “b” — fewer wild birds exist on the continent than in 1970 is a shocking figure, but the sad fact is that the article probably doesn’t come as a complete surprise to birders or even backyard bird enthusiasts. The evidence of our own eyes and ears confirms the details of the comprehensive study reported in the pages of Science. There are fewer birds, which has been becoming painfully clear over the past few decades.

I first got into birding in 1993. Now, 26 years later, I have noticed some of the declines in just the past quarter of a century. Every autumn, the variety and numbers of migrating warblers that visit my yard has gone down.

The new study in Science focuses on the drop in sheer numbers of birds. According to most experts, the bird population in the United States and Canada was probably around 10.1 billion individual birds nearly half a century ago. That number has fallen 29 percent to about 7.2 billion birds, an alarming loss of nearly 3 billion birds just in North America.

I have personally noticed signs of this dramatic loss. Let me share some personal anecdotes. These stories don’t serve as definitive proof, but they add to my unease about the state of our feathered friends.

For one thing, I no longer host large flocks of birds at my feeders during the winter. One would expect birds to mass in sizable flocks in the vicinity of feeders during a season when resources can be scarce. In the 1990s, I hosted flocks of pine siskins and evening grosbeaks that numbered in the hundreds and dozens, respectively. At times, large flocks of American goldfinches, purple finches and house finches flocked to my feeders, too. I haven’t seen an evening grosbeak since 2001. Pine siskins still visit, but I consider myself fortunate to host a flock that numbers a dozen or more.

Photo by Bryan Stevens • A pine siskin in a spruce at Carver’s Gap on Roan Mountain.

The sights and sounds of summer have changed, too. Two of the most dependable summer songsters used to be Northern bobwhite by day and Eastern whip-poor-will after dark. I haven’t heard a whip-poor-will at home in more than 20 years. The last time I heard a Northern bobwhite was about a decade ago. I live in a rural area that used to be fairly agricultural. The disappearances of bobwhite quail and whip-poor-wills is reported throughout the ranges of these two species.

Of course, there are always exceptions. Some birds have grown even more common in recent decades. Regionally, look at birds like great blue herons, double-crested cormorants, and cliff swallows, which have also shown an increased presence.

What killed the passenger pigeon? People did. What’s caused the precipitous drop in bird numbers since the 1970s in North America? Once again, people must shoulder most of the blame. We have destroyed or altered habitats essential for birds to thrive. We’ve paid little attention to the signals from some of these kin of the proverbial “canary in the coal mine” that something’s wrong in nature.

Yet “hope is the thing with feathers,” as the poet Emily Dickinson phrased it, and the losses are a signal to pay attention, not to panic. Birds are amazingly resilient. Birds need only the same things as humans — food, shelter, water. Well, perhaps they need one more thing. Birds require a safe and welcoming space in which to unfurl their wings and fly.

The great flocks of passenger pigeons may be no more, but there’s no reason to think our remaining birds can’t continue to soar, so long as we provide them with their essential needs and offer them a degree of protection and compassion.

An artist sketched this scene of hunters firing on one of the last great flocks of passenger pigeons.