Most passerine (songbird) species migrate at night. Conditions are more favorable at night; temps are cooler, the air is usually calmer and there are fewer predators. Neotropical migrants that migrate during the day (diurnal migrants) are hawks and vultures — birds that soar. These soaring birds migrate by finding thermals (rising currents of warm air) where they can hitch a ride while conserving energy. Most swallows and swifts are also diurnal migrants because they feed on flying insects that are active during the day.
Weather conditions often dictate the altitude of migrating birds. Most raptors stick pretty close to terra firma because that’s where the thermals are. However if they catch a good thermal they could rise more than a mile high. Nocturnal migrants with a good tailwind migrate at higher altitudes because the wind is stronger at higher altitudes.
Conversely, if they have to buck a headwind they migrate at lower altitudes. Most passerines migrate at altitudes between 500 and 2,000 feet. The highest known migrant was a Ruppell’s griffon vulture recorded at 37,000 feet, and the North American record holder is a mallard that, unfortunately, collided with an airplane at 21,000 feet.
Birds use a variety of factors to navigate. Some navigating tools include stars, the earth’s magnetic field, topographic features (coastlines, mountain ranges, etc.) the location of the setting sun and prevailing wind patterns.
And one thing birders know about migrating songbirds — a “fall out” can be dizzying. Many different species often flock together during migration, and after a long night of flying they’re pretty hungry in the morning. These mixed flocks can be found foraging together in the trees and understory. Trying to pick out individual birds to I.D. when there may be 20 in your field of vision all fluttering and/or hopping about requires a little practice and patience.
Identifying fall migrants adds another challenge too. Fall plumage can be quite different from breeding plumage, and during migration there will be individuals in different stages of molt. Some may still be in breeding plumage, while others may be already sporting winter plumage and many will be somewhere in between.