Success in birding often hinges on being in the right place at the right time. In the pre-dawn dark of September 21, I sat alone at Fish Beach on Monhegan Island. A bulging Harvest Moon illuminated the harbor and outlined a small fleet of tuna boats anchored to the south. Why was I on the beach? With clear skies, I hoped to spot some birds crossing the glowing orange surface of the moon. Occasional “chip” notes overhead confirmed my suspicions of movements aloft. Although night travel is an essential component of seasonal migration, not all birds migrate at night. Generally speaking, soaring birds, raptors and the like, almost always migrate in daytime to maximize the benefits of thermal updrafts. Once attaining sufficient altitudes, raptors get a gliding free ride over many miles.

The majority of birds employ powered flight techniques, however. These flights usually begin about thirty minutes to an hour after sunset. There are exceptions, of course, linked to special circumstances. Shorebirds at inland sites initiate their long-distance migration after sundown; at coastal sites, flocks leave on the high tide, since they can no longer forage for food. For one-ounce semi-palmated sandpipers, it’s an epic nonstop journey to the West Indies or northern South America that continues for 50 to 80 hours.

We might wonder why songbirds choose to migrate at night? Several plausible theories are given. Under the shield of darkness, small birds might be less susceptible to predatory attacks. And since sustained flight generates body heat, the night air is cooler, more laminar and less turbulent than daytime air. The structure of the atmosphere can be very complex at times. This includes vertical and horizontal air movements, and altitudinal differences in temperature, wind strength, wind direction, and humidity. Among these, turbulence, gradients, gusts, shears and thermal updraft are most important.

Frontal weather systems are often correlated with large movements of birds. In fall, intense cold fronts trigger migration by signaling an end to many food sources. Additionally, fronts produce northwest tailwinds winds that propel migrants south, increasing their ground speed. Migrant numbers often dwindle in the days following a front.

Back at Fish Beach around 5:30 a.m., night-roosting herring gulls launched their piercing, clamorous calls from the nearby Nigh Duck ledges. Individual gulls trickled into the rising air columns, creating smudgy silhouettes against the brightening skyline over Manana Island. And guess what? The flight paths of a few gulls trailed across the face of the setting moon!

Beyond my meager efforts at monitoring night migration, there are more convenient and reliable ways to visualize seasonal bird movements at your home on a real-time basis. BirdCast Real-time analysis maps show intensities of actual nocturnal bird migration as detected by the U.S. weather surveillance radar network between local sunset and sunrise hours. BirdCast surveillance radar gathers information on the numbers, flight directions, speeds and altitudes of birds aloft in order to expand the understanding of migratory bird movement. And, under favorable atmospheric conditions on particular nights, the skyways transport millions of individual birds to destinations farther south. Updated every six hours by Colorado State University and the Cornell Lab of Ornithology, the site delivers colorful, daily forecasts and live maps of continental bird movements. Fall migration tracking will end on November 15, so give BirdCast a look.