Although the spring migration was underway by early February, we are at its peak now.

Warblers, thrushes, cuckoos, flycatchers and other migratory species are streaming through Maine. Some will breed here and others will continue north.

Each species has its own migratory schedule, timed to ensure arrival when its favored food is available. Warblers, vireos, tanagers and cuckoos glean caterpillars from the leaves of deciduous trees. Arrival before leaf-out would be a recipe for starvation. Similarly, flycatchers, swifts and night-hawks have to delay their arrival until flying insects are on the wing to provide their meals. It is no wonder that most of our migratory birds arrive in May. Species that can arrive earlier have broad diets and can subsist on seeds or residual berries. Such species include red-winged blackbirds, common grackles and song sparrows.

You can see the spring arrival schedule of Maine migratory breeding birds at: hobbes.colby.edu/arrival.

How does a bird know when to migrate? Each one has an internal clock that is responsive to changes in the length of the day. When the days on the wintering grounds get longer than a critical time (or shorter than a critical time if the birds are wintering south of the equator), a bird’s internal clock induces it to start getting ready for the northward migration.

The internal clock induces a behavior called migratory restlessness, easily observed in migratory birds maintained in captivity. The bird is getting antsy to leave. A bird will also begin to feed voraciously to put on fat for the first leg of its migratory journey.

The weather can strongly affect bird migrations. Long-distance migrations are arduous enough without flying into a headwind. In the spring, good migratory flights are induced by periods of strong southerly winds to provide some tailwind for the migrants.

You can get a good idea of how many migrants you can expect to see on a particular morning by looking at a weather map.

High-pressure systems – anticyclones in meteorology-speak – have a clockwise rotation, so southerly winds are found on the trailing edge of a high as it moves across our continent. Low-pressure systems, or cyclones, rotate in a counterclockwise manner, so southerly winds are found on the leading edge.

The perfect time for migration is when a low-pressure system is pushing against a high-pressure system to the east. Both systems produce southerly winds at their intersection and, if you are lucky, the front where the two air systems meet will produce rain. Birds will migrate at night to take advantage of the southerly winds but will be forced to land by the rain. Voila – a fallout. Birds can seem to be dripping from the trees at dawn under these conditions.

Hawk watchers are well aware of this phenomenon. Spring hawk counts are conducted daily from Bradbury Mountain in Pownal. The best counts occur on days with southerly winds. When the wind is from the north, forget it.

NEXRAD weather radar can be used to monitor migrations. Migrating birds show up as blips on the radar screen. These were originally called angels before radar operators realized they were birds. You can delve into NEXRAD at aos.wisc.edu/weather/wx_obs/Nexrad.html.

A fantastic resource called Birdcast incorporates eBird observations, NEXRAD images and weather forecasts to predict the magnitude of migration all across the United States and beyond Visit birdcast.info.

I recommend starting with the link called “A Primer for New Migration Forecast Tools.” The maps are the most useful and fascinating aspect of this site. The prediction of migration intensity is made for every area of the country three hours after sundown and is updated every six hours. Spotlight links and special interest links will keep you occupied for hours. What a resource.

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

whwilson@colby.edu