Thursday, October 11, 2007

Gems traversing the sky

kw: book reviews, nonfiction, ornithology, migration, songbirds

Having lived for years in the Midwest and West, a favorite sight was the fall flocks of blackbirds. A grain field would seethe with small, black bodies, and suddenly the black mass would leap overhead, darkening the sky in what the Danes call Blacksun.

I knew that they would soon disappear, the next time a North wind arose, suddenly heading out at dusk toward Central or South America. Blackbirds are opportunists, and don't go farther than they must. Some stop in southern Texas, but in a dry year they'll all wend their way much farther, perhaps to Argentina.

The seemingly subtler migration of the more colorful, gemlike warblers and thrushes and finches often occurred without my notice, unless I happened to be out, perhaps walking after dark, or finding one more Messier object with my telescope.

On only one occasion while peering at the Moon did I see a barely-perceived, dark shape cross it. I realized later I'd seen a small bird, still small at 60x from two miles away, flicking across the quarter-degree-wide half-cookie of the moon in a second or less.

Mostly I just heard them; if I happened to listen stilly, I'd hear their faint twitters. It never occurred to me to try to record the sound. It has occured to others, and such recordings, computer-parsed and analyzed, are a powerful tool to discern songbird populations as they change from year to year. I like the idea. It's so much less intrusive than trapping a bird to glue a radio transmitter to the nape of the neck.

Miyoko Chu of the Cornell Lab of Ornithology writes of these and other methods for learning the hidden lives of migratory songbirds in Songbird Journeys: Four Seasons in the Lives of Migratory Birds. But even more, she writes of the birds. How tiny half-ounce chickadees or one-ounce warblers will fly a few hundred miles overnight, losing a quarter or more of their body weight along the way. No wonder they descend on the fields like a ravenous horde...they are ravenous!

I hadn't read, anywhere before, that all migratory birds nest in the North and, though it is Summer in the South while they are there, do not nest there, and that the birds that do nest there do not migrate. I wonder if this is a purely Americas phenomenon; the book focuses on American species. Perhaps South African birds that nest near the Cape migrate north to have a European non-nesting sojourn.

Each of the book's four sections has a descriptive, evocative portion, focusing alternately on specific species and on the larger phenomena of the season. Each also has a closing section on times and places at which one may see great concentrations of migrating birds (two seasons), nesting birds (Summer), and overwintering birds, plus tips for observing and recording bird behavior in your yard or neighborhood.

The most fruitful tip: few birds fly far on still days, very few when it is very windy, and the largest numbers on seasonably windy days. Particularly, wind on a cooling Fall day is spiraling from a high, which means winds out of the North if it to your West; on clear days in Fall, migrants take advantage of the tailwind. Wind on a rainy day in Spring is running ahead of a warm front, blowing out of the South as stormy weather approaches from the West. Spring migrants will risk the storm to get a good tailwind. A sudden shift in either kind of weather will yield a bird-fall with or ahead of the rainfall. Great days to see birds, but you must recognize that they are fleeing disaster, and probably exhausted.

One clear purpose of the book is to gain more eyes and ears, gathering some of the mountains of data needed by the rather small community of professional Ornithologists. This is one area in which the help of "amateurs" is vital. See eBird if you wish to participate.

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