The southward migration of 5 billion birds in North America is a staggering phenomenon. These migratory movements provide us with the chance to see a diversity of birds, often in very large numbers.

Migrating birds are flying on the edge of survival. Flight is taxing, requiring that a bird’s metabolic rate be raised five times or more above its resting rate. With the exception of birds like swallows, swifts and kites that feed on insects on the wing, migrating birds cannot refuel in flight. One way migrating birds try to maximize the length of a migratory leg is to take advantage of favorable winds.

A weather map offers you a tool to predict when migrations will be strong by looking at the relationship of low- and high-pressure systems. Generally, 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 has winds that flow from north to south.

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

To determine when conditions are perfect for a wave of fall migrants, find the nearest cold front (indicated on the weather map by a line with triangles). At the cold front, an area of cool air from a high-pressure system moves underneath the less dense, warmer air of the low-pressure system. The rising air cools, usually causing a line of precipitation.

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.

If you see a warm front passing across your neighborhood, you don’t need to worry about getting up at the crack of dawn to look for migrating warblers or sparrows. A warm front (indicated by a line with semicircles on your weather map) is caused when a low-pressure system slides over and above a high-pressure system. In this case, the winds at and behind the front will flow to the north. Birds migrate mostly when the winds are favorable and will stay put if the winds are opposite their migratory trajectory.

On a favorable night, birds will fly as long as they can, settling down sometime in the wee hours of the morning. Sometimes precipitation can cause birds to land en masse. By luck, a given spot may be hopping with birds.

Where is the best place to look for a fallout? Frankly, serendipity plays a huge role here. Sometime fallouts are broad, occurring over a county-sized area; at other times the fallouts may be quite restricted.

But some places are more likely to produce fallouts. Shrubby areas along the immediate coast are often productive. Birds that are migrating just offshore will seek landfall to rest and refuel. A dawn excursion to a coastal area can produce a stream of tired birds, welcomed by dry land. Fort Foster in Kittery, Two Lights State Park in Scarborough and the Eastern Promenade in Portland often harbor many fall migrants. I have experienced fallouts at West Quoddy State Park in Lubec. Any patch of coastal real estate with some bush and tree cover can be productive.

Migrating birds may look for islands of habitat in areas that are heavily developed. The Evergreen Cemetery in Portland is one such magnet.

Finally, migrants need water, so checking ponds, streams and rivers can produce excellent fall migrant numbers.

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

whwilson@colby.edu