Fall migration is upon us, including for yellow-rumped warblers, who will spread along the eastern seaboard from Massachusetts to Florida for the winter. A few hardy ones will even stick it out here in Maine along the southern coast. Mike Mulholland/MLive.com/TNS

We are now right at the peak of fall songbird migration. Warblers are streaming through in high numbers. Sparrows will peak sometime in October.

The destinations of these fall migrants vary. Some, like yellow-rumped warblers, will spread along the eastern seaboard from Massachusetts to Florida for the winter. A few hardy ones will even stick it out here in Maine along the southern coast.

Other species – including chestnut-sided warblers, Cape May warblers, blackpoll warblers, scarlet tanagers, and bobolinks – will vacate the continent to arrive at wintering grounds in the Caribbean, Central America and South America.

Songbirds generally migrate in a series of legs, covering up to 300 miles in a night’s flight. Each migratory leg is arduous so birds do all they can to make the flight easier.

The most critical feature of a good night for migration is wind direction. In the fall, winds blowing to the south are just the ticket for a relatively cheap flight. Of course, birders want to be afield early after a flight to see what birds have dropped in for a short visit.

A brief foray into meteorology will allow us to understand how birds pick migration times. Low-pressure systems and high-pressure systems move across North America from west to east, generally alternating low, high, low, high.

A low-pressure system has a counterclockwise flow of air as it moves east. So, the leading edge of a low-pressure system has northerly winds, and the trailing edge has southerly winds. The high-pressure system trailing the low-pressure system has southerly winds on the front side and northerly winds on the trailing side. So, the perfect time for migration is where a high-pressure system buts into a low pressure system. Southerly winds are generated by both systems.

The weather where the two air masses meet is often unsettled. So, birders know to bring a raincoat when conditions were good overnight for a strong flight.

In autumn in the days before the internet, birders eagerly studied the daily national weather map in the newspaper. We could watch lows and highs moving across the country and plan to be out in the field when migration conditions were favorable. Now, we can watch air masses move in real time on our phones and computers.

Radar was developed secretly just before World War II for military use. Early radar operators were mystified by radar blips at night, calling these blips angels. We now know those signals were migrating birds and bats.

With radar data freely available to anyone, we can use radar images to actually confirm major flights of birds, rather than simply predicting that a good flight is likely to occur.

Birders used various meteorological websites that provided radar images to look for migrating birds. Now, most birders use BirdCast, a fantastic tool developed by the Cornell Laboratory of Ornithology. This site is optimized for use by birders. You can find the actual count of birds that have passed over any county in the country in real time, flight speeds, altitude, and precise direction of flight.

For educators, BirdCast is a wonderful resource for classroom activities. It can be used from lower elementary grades to college-level classes. Checking BirdCast is a great family activity, too.

Sometimes, birders get up early because BirdCast shows lots of birds migrating but are disappointed by a dearth of birds at dawn. The problem is that the great flight of birds overflew you. Areas 100 miles to the north or south may have lots of migrants but your luck was poor.

On the other hand, migrating birds may encounter inclement weather and be forced to land. Then, one sees a fall-out, one of the most exhilarating birding events you’ll ever know. The birds seem to be dripping from the trees.

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