The earlier sunsets and the nip in the air at night usher in autumn. Although the cold of winter is sure to come, I welcome this season because of the phenomenon of the fall bird migration.

Fall and spring migration differ in some striking ways. Spring migration is more compressed. There is an urgency in spring migrants as they rush to get to the breeding grounds and set up shop in a prime territory. The birds are dressed in their best finery and the males sing vigorously, practicing for their performances once they acquire a territory.

Fall migration is more leisurely. Most of our swallows depart by early August and some sparrows will linger into December before moving south. Fall migrants have often molted into their dull basic plumage and males don’t sing.

Nonetheless, the fall migration has some advantages over the spring migration. More birds are found in the fall migration, thanks to the reproduction of our migratory breeding birds. Ornithologists estimate that two billion birds migrate into North America in the spring but five billion migrate south.

Rarities are more likely to be seen in fall migration. Many of these rarities are juvenile birds, embarking on their first migration. Some are navigationally challenged and end up in unexpected places. One good example is the Clark’s grebe that appeared on Togus Pond in Augusta on Aug. 8 and has delighted dozens of birders.

Clark’s grebes nest in lakes and freshwater marshes in western North America. The eastern-most breeders are in Wisconsin. These grebes are very rare in eastern North America. Maine has one prior record, an individual that spent three weeks at Owls Head in 2005.

As one would expect, the Togus grebe is a young bird.

It is possible to see some birds migrating south during the day. A minority of our migrating species are diurnal migrants. The sight of V-formations of Canada geese or double-crested cormorants lifts our spirits.

Falcons, eagles and hawks migrate during the day as well. These birds are skilled in using thermals, the vertical winds that form during the day because of uneven heating of the surface of the earth. Rocky terrain warms more rapidly than adjacent forested land. The rocks absorb heat from the sun and heat the overlying air. The air rises and is replaced by cooler air from the forested region. That cool air is heated above the warm rock and the cycle continues. These birds are adept at soaring from thermal to thermal, scarcely having to flap their wings.

Sustained flight is an energetically demanding activity. Migrating birds wait until winds are blowing in a favorable direction. So, observing hawk migration is a boom-or-bust activity. When winds are blowing from the north, a strong hawk migration can occur.

Most smaller birds migrate at night but swallows, swift American robins, blue jays and some finches are daytime migrants. Look for them as part of the diurnal fall migration.

You can predict good days for migration by looking at a weather map. Specifically, you want to look at the relationship of low- and high-pressure systems in your general area.  High-pressure systems alternate with low-pressure systems, moving from west to east across the continent.

A high-pressure system has winds that circle the center of the system in a clockwise pattern.  The leading edge of the high-pressure system therefore has winds that flow from north to south.

A low-pressure system has winds that flow counterclockwise.  The winds on the backside of a low flow to the south.

Look for the nearest cold front (indicated on the weather map by a line with triangles).  As the front passes, a strong flow of wind blowing to the south occurs with the interaction of the trailing edge of the low-pressure system and the leading edge of the high-pressure system.  Depending on the rate at which the high-pressure system moves, spectacular migrations may be seen for several days.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu

Copy the Story Link