Something massive and extraordinary, perilous and strange, is happening, now. It’s one of the great spectacles of nature, and until recently it was invisible to us. It’s happening high in the sky—in the dark, while we’re sleeping—the passage of hundreds of thousands, sometimes millions, of birds over North America every night during spring and autumn migration. Now, thanks to technologies enabling us to perceive masses of birds with weather radar, listen to tiny chirps high in the sky, and follow banded birds with special GPS systems, we can track this magnificent phenomenon, figuring out where the birds go. But how do the birds themselves figure out where to go? How and why do they pull off such long-distance journeys in the dark?

Cliff Swallows

At least 40% of the bird species in the Northeastern U.S. and Canada migrate, and some 80% of those make their trips at night.

Of the birds migrating during the day, certain species, such as the ruby-throated hummingbird, migrate both day and night in intense, non-stop journeys. Others, such as raptors, prefer to migrate during the day when they can go upwards on rising columns of warm air and then glide long distances, slowly losing altitude but expending very little energy along the way. As for the majority of migrators, they have several reasons for migrating almost entirely at night:

  • At night the air is cooler than in the daytime. For birds that flap instead of gliding to move themselves forward, the cooler air helps them avoid overheating as they exert themselves tremendously.
  • Because it is cooler, nighttime air is usually calmer than daytime air. For smaller birds, the calmer night air is helpful in keeping them from being blown off course.
  • Crucially, smaller birds are adapted to fly at night in order to avoid their predators, the raptors that migrate during the day.

An unlucky bird met an untimely end in the claws of this Cooper’s Hawk, a fate that flying at night helps smaller birds avoid during migration.

BUT—birds don’t have headlights. How do they know where they’re going at night? And how do they know where they should be going? Numerous scientific studies indicate that birds use a variety of tools to help orient themselves in flight, correct their paths when winds have pushed them off course, and navigate to desired locations at night—often to the exact same nesting spots year after year. Here’s how they do it:

First-year birds rely a lot on instinct, a set of genetically-encoded internal directions whose precise mechanisms are still mysterious to us. The young birds’ instincts are not perfect, and much can go wrong—which is why up to 30% of first-year migrators perish. It helps to travel in flocks. Flocking enables birds to share information about obscured landmarks; less experienced birds can also benefit from the knowledge of older migrating birds.

Experienced birds use several additional tools to help them find the right routes in the dark:

  • They remember landscapes they saw by starlight or moonlight during previous migrations—mental maps of rivers, mountains, and other landmarks that they have memorized.
  • They remember smells from earlier trips—the scents of stopover locations that provided good food and shelter.

This Homing Pigeon has a small area on its beak containing magnetite, a mineral that responds like a compass to the Earth’s magnetic field.

Migrating birds of all ages also use receptors in their beaks and/or eyes that help them sense the Earth’s magnetic field, a particularly useful extra sense during the night.

Birds have sensory receptors in their beaks and/or eyes that enable them to sense the north-south orientation of Earth’s magnetic field.

Because of the Earth’s rotation on its axis and how that spinning affects its molten iron core, the Earth is a giant magnet. Its magnetic field stretches vertically between its two poles like a huge sheath made of vertical lines. Quantum-level chemical reactions in birds’ eyes, activated by blue light, respond to the magnetic lines, helping the birds to orient themselves on a north-south pathway.

Thus birds at night can tell if they are heading along a north-south line as opposed to an east-west one. But how do birds migrating up from the tropics towards us in springtime know which way is north, instead of south? They sense the line, but how do they know which direction to take it in?

They make use of two more clues:

  • The setting sun shows them which way is west.
  • Birds find the North Star (Polaris) and head towards it in the springtime, and away from it in the winter. Because it is located right above the North Pole, the North Star appears fixed in one spot in the sky while all the constellations move around it. Thus it isn’t subjected to the visual changes caused by the Earth’s rotation on its axis, nor its rotation around the sun. Birds, like human sailors of old, know this trick!

Birds orient themselves to the North Star, Polaris, in order to know which way is North. Often humans find the North Star by first finding the Big Dipper constellation. From the top corner of the Big Dipper’s cup—the corner farthest from the “handle”—trace a line to find the Little Dipper Constellation. The star at the end of its “handle” is the North Star.

These are all wonderful adaptations for flying at night. But although migrating birds may seem like superheroes, their amazing abilities do not, in fact, make them indestructible—in fact, migration is a very dangerous process. Heading to better eating and breeding grounds is worth it to them, but still, their journeys can be very difficult. Some human activities make their journeys much worse:

  • Habitat loss is very perturbing. Imagine coming home at night, utterly ground down, frazzled, and worn out from a very long trip, and finding to your shock that your house is no longer there! And your local supermarket is no longer there! They’re just GONE. Now what are you going to do? Do you have the strength to go find new, distant places where there’s food? Where are you going to sleep?
  • Artificial blue lighting at night can be very disorienting to migrating birds. White light contains all the colors, including blue. Migrating birds are attracted to blue light, probably because it helps them sense the magnetic field. But if blue or white light comes from buildings, the birds are at serious risk of hitting windows and dying. The problem is especially severe in misty or foggy conditions, because then birds can’t use their regular vision to help them accurately make out where the buildings are, in the same way that it’s hard for us to see when driving in fog. When trees are reflected in windows due to artificial light (or at dawn), and there’s also fog or mist, the danger is particularly acute. Yellow light is safer for birds, especially when it’s pointed downwards rather than upwards towards the sky. No outdoor lighting at all is safest during migration season.

Notice how much of the nearby trees is reflected in this window at dusk. A bird thinks it can fly right through the glass to the tree on the other side, a misunderstanding that is very often fatal.

Christine M. Du Bois, Coordinator of the Lansdowne Bird Town and editor of Bird Beat

Image Credits:

Night Flight, by Ronnie Robertson, licensed under CC BY-2.0

Cliff swallows hunting for insects before sunrise, by Jacob W. Frank, U.S. National Park Service, Public domain

Migration Time, by Tony Alter, licensed under CC BY-2.0

Accipiter cooperii DM3, by Cephas, licensed under CC by-SA 3.0

Migrating birds flying in dusk, by U.S. Fish and Wildlife Service, Public domain

Postduif (Homing Pigeon), by Patrick Edwin Moran, Public domain

Magnetic meridians, by Sch, licensed under CC BY-SA 4.0

Emagone Constellations around Polaris, by Emagone, licensed under CC BY-SA 4.0

Region house, window, 2018 Oroszlány, by Globetrotter19, licensed under CC by-SA 3.0