The fall land-bird migration is peaking now. Most warblers have already passed south. Thrushes and sparrows will dominate the October migration.

The fall migration is more leisurely than the spring migration for most birds. There is an urgency about the spring migration, as birds are driven to arrive on the breeding grounds as early as possible to stake out good territories and find a mate.

In some ways, the fall migration is even more impressive. This migration involves the spring migrants that survived the breeding season as well as all the young born in the summer.

Ornithologists estimate that 5 billion birds migrate in North America alone each fall!

Some species of birds migrate during the day. These include pelicans, hawks, falcons, hummingbirds, swifts and swallows. 

All of these birds are strong fliers. The larger birds can take advantage of thermals that develop during the day. Swifts and swallows can feed on the wing during the day as they migrate.

The majority of land birds migrate at night. These include cuckoos, flycatchers, warblers, vireos, thrushes, orioles and sparrows. Most of these birds are denizens of woods and other sheltered habitat. These birds are not extremely agile fliers so need dense habitat to avoid bird predators.

Migration at night has at least three advantages. Birds do not have to worry about falcon or hawk attacks. Second, the air in the atmosphere is usually less turbulent than during the day.  Lastly, the air is cooler at night. 

A migrating bird produces a huge amount of excess heat that needs to be released. Most of the heat is lost from the unfeathered legs. The colder the air, the more quickly that heat can be dumped.

Migrating birds wait for favorable winds before starting a migratory leg. Keep in mind that high-pressure systems rotate in a clockwise fashion and lows are counterclockwise. So, the leading edge of a high or the trailing edge of a low have the southerly winds that favor a strong migratory flight that night.

The evidence of a strong migration can be a fallout of many birds the following morning. But there are more direct ways to experience nocturnal migration.

First, you can set up a spotting scope or use binoculars and watch the face of the moon.  It’s very cool to watch migrating birds wing across the lighted surface. Most nocturnal migrants start shortly after dusk and peak around midnight.

Migrating birds can also be seen on radar. In the early days of radar, echoes of many, small targets were seen but poorly understood. These echoes were called “angels.” Now we know that the angels are actually birds and sometimes bats.

Radar is now used as a powerful tool to study nocturnal migration. Sid Gauthreaux of Clemson University is one of the pioneers of this field. Check out the radar image showing many angels at his website: http://virtual.clemson.edu/groups/birdrad/COM4A.HTM.

Yet one more way to appreciate nocturnal migration is to use your ears. Nocturnal migrants are noisy, regularly emitting short flight notes. In some cases, the flight notes are similar to the calls the birds give while they are on the ground. In many cases, however, the flight notes are only given during a nocturnal flight.

Bill Evans has been a pioneer in the study of nocturnal flight calls. Visit his website at www.oldbird.org/. Evans also sells a CD with recordings of the nocturnal flight calls of 211 species. It’s a good investment.

On a night that is not too windy, you can hear the flight notes above. However, a microphone will capture many more of those vocalizations. Evans provides directions on how to build a microphone system using cheap materials like a plastic flowerpot, saran wrap, a dinner plate and an inexpensive microphone. 

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at:

whwilson@colby.edu