South for the Winter

Sometime in mid to late August, the first wave of migrating warblers moves through our yard. They come in early morning to feed after flying much of the night. Their appearance signals the passing of the first cold front with a north wind and clear skies that helps them fly more quickly and easily. This push sometimes doubles their speed.

Migrating birds use innate compasses to find their way to both their summer and winter homes. The night migrants have one based on the stars and the day migrants one based on the sun’s position. The immature songbirds going South for the first time not only use their innate knowledge of time and distance, but what they learn on their own during that first flight.

According to scientists who have been studying bird migration for decades, night migrants teach themselves the positions of the stars, particularly that of the unmoving North Star and the spatial relationships among the constellations. They also depend on the patterns of polarized light in the sky at sunrise and sunset to identify true north. In addition to sensing earth’s magnetic field, both day and night migrants may be able to sense the low-frequency sound waves made by the ocean’s surf and trade winds.

Changes in day length signal their internal clock that it is time to begin migration. Night migrants become restless when it is dark, and all migrants start changing their diets by eating foods that help them build up their normal three to five percent body fat to as much as 30 to 50 percent in migratory songbirds. This energy-giving fat allows them to travel as far as 620 miles without eating.

Five billion birds migrate in the Western Hemisphere, heading primarily for the lowlands of southern Mexico and Central America, followed by the West Indies and northern South America. Most small birds are night migrants, traveling singly and flying below 2000 feet at 20 to 30 miles an hour. They take off usually between 30 to 45 minutes after dark and land long before dawn.

Why they migrate at night is not so clear, but researchers theorize that they are better able to feed and build up reserves during the day. Also, the air is calmer and it is cooler at night so the migrants are not as likely to dehydrate.

Day songbird migrants are swallows, swifts, robins, flickers, hummingbirds, and the jay family, although why they prefer to fly during the day is not clear either except that swallows and swifts catch insects on the wing. They start shortly after dawn and reach a peak at 10:00 a.m., flying in flocks instead of individually like the night migrants, but flying at the same speed and height.

Raptors are also day fliers because they rely heavily on soaring flight, which saves them a lot of energy. Clumps of warm, rising air or thermals from mid-morning to mid-afternoon move them along swiftly and sometimes as high as 5000 feet. Shorebirds and waterfowl, which learn migration routes from their parents and the rest of the flock, mostly fly during the day, but they fly as high as 15 to 22 thousand feet at 30 to 50 miles an hour.

There are three kinds of migrants. Complete migrants entirely leave their breeding range and migrate to a separate winter range. Examples include black-throated green warblers, rose-breasted grosbeaks, and chimney swifts. Partial migrants leave only a portion of their breeding range, for instance, song sparrows, robins, and red-tailed hawks. Irruptive migrants are not seasonally or geographically predictable but follow food sources. Good examples in our area are evening grosbeaks, pine siskins, and common redpolls.

There are also three kinds of migration. Most common here is the north/south migrants. Some, though, travel east/west, most notably our tundra swans. And, in the western United States, several species migrate up and down high mountains.

Every autumn I can depend on seeing migrants, but the cast of characters varies. Still, there are some dependable migrants. Black-throated green and yellow-rumped warblers are always our most common warbler species, followed by black-and-white and black-throated blue warblers. Last September I also saw a Wilson’s warbler on September 19 and a Nashville on September 25.

But October is our peak autumn migration month. October 3 brought me both a magnolia and golden-winged warbler as well as red-breasted nuthatches. The following day white-throated sparrows and ruby-crowned kinglets appeared for the first time since the previous April. The first wintering dark-eyed juncos also arrived that day because at the same time our summer visitors are leaving, our winter visitors are replacing them.

By mid October, most of the Neotropical migrants–scarlet tanagers, ovenbirds, wood thrushes, etc.–are gone. Those birds that only migrate to the southern United States–hermit thrushes, yellow-rumped warblers, eastern towhees, eastern phoebes, blue-headed vireos, for instance–stay longer, and every year I record here at least one of those species in early November.

Raptor watching is another highlight of October. Besides using thermals, raptors also use updrafts made by wind that is deflected off ridges. The longer the ridge and the steadier the wind, the farther a raptor can fly with little use of its own energy. Our ridge, part of the Bald Eagle Mountain, which is the westernmost in the ridge-and-valley province, may not rival Hawk Mountain’s migration, but there are days when sitting on top of First Field is rewarding. Mostly, I see dozens of sharp-shinned and red-tailed hawks, along with turkey vultures, but sometimes I am surprised by more spectacular species.

Last October 18 it was windy and clearing after a cold front that had moved in over night. Lying on top of First Field, watching the shapeshifting of kaleidoscopic clouds, I also watched a parade of red-tails and sharpies zipping past high in the sky. Although i was earthbound, I felt as if I had been lifted into another realm of wind and sun and clouds.

Then, suddenly, I spotted a much larger, dark bird sailing past high above me. From below, the only distinguishing marks were the flashing white splotch on each wing and a long white tail rimmed with black. It was an immature golden eagle.

According to the recently-published The Birds of Pennsylvania by McWilliams and Brauning, back on November 6, 1990, 22 golden eagles flew past the Bald Eagle Mountain Fire Tower with is 14 miles northeast of us. Sightings there continue to make our mountain one of the best places to see golden eagles in migration.

Seeing the golden eagle was the highlight of my October migration watching, but November brought its own exciting migrants. On the first of the month, tundra swans called so high in the sky I couldn’t see them. I also put up my bird feeders.

Five days later three pine siskins arrived. I held out hope throughout the month and into December that it would be another irruptive year because a couple pine siskins appeared off and on and even common redpolls once in mid-December. But it was not to be.

On the other hand, winter residents such as tree sparrows, which began arriving on November 9, were in higher numbers than usual. We also had more frequent sightings of purple finches and a wider selection and number of partial migrants that remained here–white-throated and song sparrows, robins by the hundreds, bluebirds, red-tails, and sharp-shinned hawks. The sharpies have been staying north in larger numbers because of the easy prey at bird feeders.

A male kestrel also made infrequent appearances over the winter after arriving in late September to hawk for grasshoppers. Female kestrels migrate three weeks to a month sooner than males and those that winter in Pennsylvania are always males as we discovered.

My best migrant sighting, though, occurred on a clear and breezy November 8. I walked over to Greenbriar Trail in the midst of the eight-year-old recovering clearcut we purchased after the damage was done. It was full of birds scratching among fallen wild grapes–white-throated sparrows and robins by the dozens. Then, in one group, I noticed blackbirds too. They didn’t look like either European starlings or red-winged blackbirds, so I examined them more closely. Ten rusty blackbirds mingled with the others, eating grapes.

These birds, which breed mostly in the boreal woods of Canada and the northern United States, are black with yellow eyes during the breeding season. But they are named for their rusty color change in fall and winter. The females also display a broad, buffy eyebrow and more bronze on their underparts than the males which retain more black and only a faint eyebrow.

I rarely see these birds during migration, but I had no idea that they have shown the greatest population decline of all bird species in the last 100 years. Since they are, after all, only blackbirds, not too much attention had been paid to them until ornithologists Russell Greenberg and Sam Droege, using a variety of bird surveys to assess their population, discovered a 92.8 percent decline in Breeding Bird Surveys from 1966 to 1996, an 89.6 percent decline on Christmas Bird Counts from 1958 to 1988, and a 92.1 percent decline between 1970 and 1995 on the Quebec Checklists Program. They also looked at 84 published state and regional accounts of the birds from pre-1920 to after 1980 as well as other published checklists, regional summaries, and data from the Migration Card Program that ran from the 1880s through the 1940s and found that the decline began in the early part of the twentieth century but increased rapidly in recent decades, particularly the 1970s.

Why rusty blackbirds have declined so precipitously is not clear. That’s because so little is known about their breeding ecology and behavior that ornithologists must learn more about these overlooked birds before drawing any conclusions about their decline. However, they speculate that since rusty blackbirds like wooded wetlands throughout most of the year, the loss and degradation of that habitat may be the answer.

On the other hand, since much of the checklist data was collected in refuges where the habitat is still good, that can’t be the whole answer. Neither can the acidification of their habitat, which may kill their invertebrate wetland prey, or the logging of wetland forests and the construction of large hydroelectric dams that flood boreal forest habitat. No doubt a complex of reasons will emerge if enough research is done on these handsome birds.

Knowing all this, I treasured my sighting of ten rusty blackbirds eating wild grapes on our dry mountaintop. Such discoveries are what make migration-watching my favorite outdoor activity in autumn.