The migration of birds ranks among our planet’s most conspicuous, awe-inspiring natural phenomena. From nearly every corner of the globe, birds as diverse as hummingbirds and herons, geese and grackles undertake twice-annual flights between their nesting and winter quarters. Yet the great majority of migrants pass overhead in darkness, unseen by human eyes.
The long-awaited spring songs of wood thrushes and Baltimore orioles materialize suddenly – and magically – at dawn in our forests each early May. The American woodcock peenting in field hedgerows at dawn and dusk, then launching into its theatrical courtship flight, seems to have appeared from nowhere, as does the common yellowthroat singing its witchity-witchity from a willow thicket. All arrived at first light, after finding their way through the night skies.
While some birds, particularly strong fliers such as raptors and swallows, migrate by day, nocturnal migrants have many advantages. The atmosphere tends to be more stable, making it easier to maintain a steady course. Cooler nighttime temperatures help prevent overheating caused by the exertion of sustained flight. Flying at night minimizes risks from diurnal predators such as hawks and falcons. Celestial navigation via the stars and moon helps birds orient properly. Finally, moving at night allows migrants, most of which need daylight for foraging, to refuel and replenish vital energy reserves.
Nocturnal migration does occasionally reveal itself to human eyes. Most conspicuous are the dawn descents – usually of songbirds – which sometimes occur along coasts and lakeshores following a night’s flight. Extreme versions of these events typically involve adverse weather fronts that ground exhausted birds. Pushed off their intended routes, and hesitant to cross large water bodies, disoriented migrants seek to reorient themselves at first light, occasionally with spectacular effects. A famous episode occurred in late May 2018 on the St. Lawrence River’s north shore, as five awestruck birders beheld an airborne flood of migrant warblers beginning at dawn. Over several hours, unimaginable numbers of northbound migrants poured by, some even flying between the observers’ legs! Every one of the 721,000-plus individuals tallied on that epic day, including a staggering 144,000 bay-breasted and 108,000 Cape May warblers, had been actively migrating the previous night.
Nearly as dramatic “fallout” events can occur when nocturnal migrants are attracted to and confused by lights on towers or tall buildings. On a night of heavy migration, thousands of birds may strike these structures and die, a hazard that has prompted many cities to enact far-reaching measures to reduce risks, such as New York City’s annual “Lights Out” campaign during periods of heavy migration in spring and fall. A different kind of nocturnal fallout occurs on offshore islands, which can provide critical refuge for exhausted migrants. For example, Machias Seal Island, a tiny (20-acre), isolated, treeless, fog-shrouded island in the northern Gulf of Maine, routinely experiences massive nighttime avian fallouts, especially during spring migration. Longtime lighthouse keeper Ralph Eldridge has witnessed and chronicled these events, which tend to coincide with migratory pulses (after adverse weather has stalled migration for up to several days) and conditions of dense fog or drizzle, which make for poor visibility. The lighthouse can be literally festooned with birds – Ralph estimates thousands on the heaviest nights – all exhausted and energetically stressed. Many continue on when the weather turns favorable, but some don’t survive. Migration is a risky proposition in the best of times.
Short of events such as these, the hundreds of thousands – sometimes millions – of migrants that blanket our skies on any given spring or autumn night may be largely invisible, but they are not silent. Most actively migrating birds vocalize while on the wing, uttering distinctive night flight calls that may sound nothing like their familiar breeding songs, or even their daytime contact calls. Often barely discernible to human ears, these chips, tweets, and buzzes – flowing overhead in the night skies – are the most tangible evidence of this unseen, seasonal phenomenon. Scientists and amateurs alike now record these nocturnal flight calls, or NFCs, via specialized acoustic monitoring devices that enable specific identification of the passing migrants.
The nocturnal migration of birds is a mysterious and stirring marvel, but one that each of us can experience firsthand. This spring, pick a clear evening with light southwest winds, ideally with a moon that is full or nearly so. Walk to a quiet, open spot and just listen. You will hear the migrants passing overhead: thrushes, sparrows, warblers, tanagers, grosbeaks. With patience and luck, you may even see the silhouette of a migrant as it crosses the moon.