Flock of redwings

A flock of redwing blackbirds gather for migration.

This morning I looked out the kitchen window over my breakfast bowl of cereal and saw a few yellowish-green birds flitting about in the overgrown copse of lilacs in our front yard. I couldn’t identify them through the screen and without my glasses on or my binoculars at hand. But they moved and behaved like warblers; they were all nervous, abrupt movement and they never dropped down to the ground to forage, but instead charged from twig to twig looking for insects stunned by the weather on this cold morning.

Bird migration fascinates most people who like birds, and there are likely many people who only notice birds during migration. There are two reasons for this. First, during migration you see birds that are strange to you; we just don’t see some of them during the rest of the year, so they aren’t part of the wallpaper. Some of these are small but flashy birds like the white-crowned and the fox sparrows.

Second, during migration some species gather into large or even huge flocks. Folks who don’t usually notice waterfowl or shorebirds are sometimes rather struck by the sight of hundreds or even thousands of them gathered on a lake or along the beach. Open country species like blackbirds, including bobolinks, also gather in large flocks to migrate.

Migration, however, isn’t what it used to be. When I began seriously watching birds nearly half a century ago there were simply more birds. In these columns I often cite the North American Breeding Bird Survey, which has published population changes between 1966 and 2014 (and is presumably ongoing), and the numbers are, all too often, on the decline.

According to the U.S. Fish and Wildlife Service, of the 1,027 species protected by the Migratory Bird Treaty, 92 species are either threatened or endangered and an additional 274 populations are declining fast enough to be listed as being “birds of conservation concern.” Many others are simply declining more slowly.

Some of these losses are due to habitat loss on breeding grounds, but they are also incurred either during migration or on wintering grounds. As you might imagine, this has been studied systematically by scientists. The Fish and Wildlife Service has gathered together numbers from several published studies (www.fws.gov/birds/bird-enthusiasts/threats-to-birds.php).

Collisions with man-made structures collectively cause the most mortality. The median annual average number killed by collisions with buildings is 599 million. Collisions with vehicles account for 215 million bird deaths per year. Electrical lines claim 26 million and communication towers 6.6 million.

The hew and cry over wind turbines is rendered somewhat absurd by the actual numbers: 234,000 per year. This number will obviously go up as the number of turbines grows, but engineering solutions to alert migrating birds to the presence of the rotating blades are already being developed. Numbers are not yet available for off-shore wind turbines, but it is likely that studies will be mandated as part of the environmental impact statements for these projects.

Cats are the largest single cause of death to migrating birds. These mid-size Eurasian predators, which are allowed to roam at will over much of the North American and European landscape, kill 2.4 billion migrating birds each year. The house cat population in the United States has grown from 30 million in the early 1970s (when I started birdwatching) to about 90 million in 2010, according to estimates derived from sales of pet food. This, of course, does not account for feral cats. Fifteen years ago, a National Geographic article cited an estimated U.S. population of 70 million feral cats. Cat advocates deny the importance of cat predation on bird populations, but the evidence is overwhelming.

Last week we were walking on the Sullivan County Farm land on Carroll Brook Road in Unity, which is fenced. As we followed the trail along the fence line, we noticed not one but two dead Swainson’s thrushes, unblemished but for their broken necks. It wasn’t until our running puppy charged straight into the chicken wire that I realized how these migrants had met their end while foraging in unfamiliar territory.

As a consequence of all this, I no longer see the trees filled with twittering warblers as I did in the springs and falls of my youth. I have an indelible memory of waking up one spring morning in the Carroll Gardens neighborhood of Brooklyn, where I lived in the mid- to late 1980s, and the tree outside my window was absolutely filled with chestnut-sided warblers. (One fall evening as I walked home from the subway I spotted a saw-whet owl sitting on a low branch of a street tree just outside my brownstone.)

Migratory birds feed with a particular intensity; they are incredibly hungry, apparently. You would be too if you had just flown hundreds of miles in the dark and you only weighed a few ounces. Thrushes provide a good example of the motivation for migration. Yes, it gets incredibly cold and dark in the far north, so that is one reason to vacate those premises, but birds migrate south from places that barely seem to have winter. Much of it is due to a seasonal lack of food. Thrushes and warblers, for example, eat mostly insects and other invertebrates, which either disappear entirely or become too rare in the winter months to support a population of birds and the young they just raised.

Fall migration began in late August when shorebirds began to gather and move southward and the loons I watch for at Otter Pond disappeared. But now is the very height of the event. One a warm clear night, sit out in your backyard and listen. Overhead you will hear the honks of geese of course, but you might be surprised at the number of whispered chips and peeps that drift down from the smaller birds who are just trying to keep track of one another.

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