Still more on migration: Many different paths

Written by Jack Sanders
Wednesday, 25 November 2009 10:59

In the spring and perhaps more commonly in the fall, we spot flocks of birds flying overhead and think to ourselves, they must be migrating. That may be true, but we never even see most of the birds that migrate through this area. That’s because they fly by night.

In general, birds that rely solely on “powered” flight — that is, constantly flapping their wings — tend to migrate in the dark while those that do some gliding or soaring, travel by day.

Daytime would seem the simplest time to travel since visibility is good and most birds are active in the daytime anyway. However, the sun that provides the light also creates heat, and a bird that is flying hundreds of miles is already hot from burning so much energy. The added heat of the sun would quickly dehydrate these migrants.

Hawks, swallows, cranes, and storks travel by day to take advantage of thermals, rising columns of air created by the sun’s heating the earth; thermals help keep them aloft. However, these same currents would be disturbing to a long-distance powered flyer, adding to the effort it takes to move through the atmosphere in a straight line. Thus, nocturnal flight tends to be smoother, with less energy wasted on combating currents.

Nocturnal flight also allows migrants time to search for food in the daylight, their usual foraging time.

There have been other theories advanced about nighttime flight, such as that fewer predators are around. That one, at least, has been pretty much been discounted by scientists.

North-south

The vast majority of the world’s birds migrate from south to north in the spring, and vice versa in the fall.

Inspect a globe and the reason becomes clear. The Northern Hemisphere has much more land mass than does the Southern Hemisphere. In our slice of the world, many more migrants that winter south of and around the equator will head for the “wide open spaces” of United States and Canada than for the narrow tail of southern Argentina and Chile. In fact, while many birds that winter south of the equator fly north for the summer, not one species that winters north of the equator flies south for the summer.

Not all birds migrate north-south. A few may move east-west. Blue-winged Teals that breed in Plains states like Kansas and Nebraska may fly east to spend the winter on the coast of Virginia or the Carolinas. Purple Finches that summer in northern Michigan may winter here in New England. The White-winged Scoter may breed in western Canada and winter on the East Coast of Canada.

Three kinds

Migration isn’t as simple as going from one area of the north to one area of the south, and vice versa. In his book, How Birds Migrate, Paul Kerlinger describes three kinds of migrations: Complete, partial and irruptive.

In complete migration, all members of the species have separate breeding and nesting grounds, usually far apart. Birds that winter in Central or South America and breed in the Northeast practice complete migration; these include our House Wrens, Ruby-throated Hummingbirds, and Wood Thrushes,

In partial migration, the most common form, some members of a species move away from summer breeding grounds while others remain. In our neighborhood, some robins and Eastern Bluebirds fly south in the fall while others stick it out during the winter. Partial migration usually results in overlapping ranges and if you look at a range map in a field guide, you will see a breeding range, a winter range, and a “year-round range.”

Irruptive migration occurs when a species that normally inhabits a region year rounds decides to burst from its normal territory and invade another. Two winters ago, for instance, we had an irruption of Pine Siskins that would normally have spent the season farther north.

Coming Up

The Hawk Season’s End, report on this season’s raptor count by Arthur Green, Bedford Audubon Society’s Hawk Counter, Tuesday, Dec. 1, 7 p.m., Westmoreland Sanctuary, 260 Chestnut Ridge Road, Bedford Corners, (914) 666-8448, awgreen @ bedfordaudubon.org,  bedfordaudubon.org

Field Trip to Edith Read Sanctuary and Marshlands Conservancy, Rye with Tait Johansson, of Bedford Audubon, Wednesday, Dec. 2, 9  a.m. (carpool from Bylane Farm, Katonah) jebecker @ bedfordaudubon.org, (914) 232-4806, bedfordaudubon.org

Bird Walks with Luke Tiller, to register, sunrisebirding.com/walks.htm; 203-981-9924, This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

First Sundays, birding at Greenwich Point with Meredith Sampson of Wild Wings, and other guides, meet at the second concession stand, 203-637-9822.

Copyright (c) 2009 by Jack Sanders. Send sightings or comments to: jackfsanders [at sign] yahoo.com, or to Bird Notes, Box 1019, Ridgefield, CT 06877; or call 203-438-1183, extension BIRD (2473), and leave a message with your report, spelling your first and last names and telling us your town. If you need help identifying a bird, try your local nature center. If you find an injured bird, call wildlife rehabilitator Darlene Wimbrow of Redding, 203-438-0618, Wildlife in Crisis of Weston, 203-544-9913, or Wild Wings of Greenwich, 203-637-9822. The columnist’s website is www. sandersbooks. com.

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