You turn off the lights, crawl into bed, and assume the backyard has gone quiet for the night.
The chickadees have disappeared. The cardinals have called it a day. Even the hummingbirds have finally stopped battling over the feeder.
But look up.
High above your house, an entirely different shift may be getting underway.
While much of the human world sleeps, huge numbers of birds take to the night sky as they migrate. On particularly favorable autumn nights, the movement can be staggering. According to the Cornell Lab of Ornithology, more than 1 billion birds can be in the air over the United States on a single night when migration conditions are right.
And yes, some of them may be flying directly over your house.
Ruby-throated Hummingbird: Before migration, Ruby-throated Hummingbirds enter a period of intense feeding called hyperphagia, building crucial fat reserves to fuel their journey south. Some may nearly double their body weight before migration, storing the energy they’ll need for long flights, including the remarkable nonstop journey some make across the Gulf of Mexico.
The Great Bird Migration Most of Us Never See
When we think about fall migration, we tend to picture geese flying overhead in a V or hawks streaming past a mountaintop.
But much of bird migration happens in darkness.
Many songbirds migrate primarily at night. Warblers, thrushes, vireos, and other migrants may spend their days resting and feeding before taking to the sky again after sunset.
Other birds follow different schedules. Hawks and other soaring birds generally migrate during daylight, when rising columns of warm air called thermals help carry them along. Swallows and some waterfowl also migrate during the day.
For many smaller birds, however, nighttime offers some advantages.
The air is generally cooler, which can help birds dissipate the considerable heat generated during sustained flight. Atmospheric conditions can also be calmer, and migrating under the cover of darkness may reduce exposure to some predators.
Then there is navigation.
Scientists have discovered that birds have an extraordinary collection of navigational tools. Depending on the species and circumstances, they can use the sun, stars, Earth’s magnetic field, and patterns of polarized light to help orient themselves.
Research has even shown that some nocturnal migrants can use the rotation of the stars to establish a north-south orientation.
Think about that the next time you’re struggling to find your car in a parking lot.
Veery: Veeries are among the many songbirds that migrate long distances at night. Unlike migrants that alternate between flapping and coasting, Veeries may flap continuously throughout an entire night’s journey. Their efficient wings help them cover long distances, including transoceanic routes, on relatively small fat reserves.
We Can Actually See Migration Happening
Here is where the story gets even cooler.
Scientists don’t have to stand outside all night with binoculars pointed toward the sky to know birds are passing overhead.
They can use weather radar.
The same radar network used to track rain, thunderstorms, and other weather can also detect biological activity in the atmosphere, including masses of migrating birds.
BirdCast, a project developed by researchers at the Cornell Lab of Ornithology, Colorado State University and the University of Massachusetts Amherst, uses weather-surveillance radar and other data to forecast and measure bird migration across the contiguous United States.
During migration season, BirdCast’s Migration Dashboard can provide estimates of how many birds crossed a particular county overnight, along with information about their direction, speed, and altitude.
Enter your county into the BirdCast Migration Dashboard, and you can see an estimate of how many birds crossed your area the previous night.
In other words, you can go to bed, wake up the next morning, and discover that you had a whole lot more overnight visitors than you realized.
Gray Catbird: Gray Catbirds are nighttime travelers, migrating primarily after dark as they journey between their northern breeding grounds and southern wintering areas each spring and fall.
So Where Do All Those Birds Go in the Morning?
They need a break.
After traveling through the night, migrating birds descend into suitable habitat where they can rest and replenish the energy needed for the next leg of the journey.
That habitat doesn’t have to be some remote wilderness.
Migrants can turn up in parks, woodlots, forest edges, and other habitat patches. During migration, that can give birders opportunities to see species that don’t normally spend the summer or winter in their area.
Depending on where they live and when they’re watching, birders may encounter migration-season visitors including Blackpoll, Magnolia, Bay-breasted and Cape May Warblers, Swainson’s Thrushes, Philadelphia Vireos and Yellow-bellied Flycatchers.
That’s part of what makes migration birding so addictive.
You can visit the same patch of woods two mornings in a row and find a completely different cast of characters.
Yesterday: nothing unusual.
Today: Where did all these warblers come from?
The answer might be: the night sky.
Yellow-rumped Warbler: Yellow-rumped Warblers are short- to medium-distance migrants, traveling between northern and mountainous breeding grounds and wintering areas farther south. Their unusual ability to digest waxy berries helps them arrive earlier in spring and linger later into fall than many other warblers.
Fall Migration Starts Earlier Than You Might Think
Another surprise is just how early the southbound movement begins.
We humans tend to associate fall migration with sweaters, pumpkins, and leaves changing color.
Birds didn’t get that memo.
For some species, southbound migration begins while we’re still firmly in summer. Shorebirds can begin moving south surprisingly early, and by August, migration is already underway for a variety of birds.
The timing varies considerably by species. Some birds are early travelers, while others don’t reach peak migration until September or October.
So don’t wait for the leaves to change before paying attention.
The birds are already moving.
There’s Another Way to Know Birds Are Up There
Go outside after dark and listen.
Some migrating birds make short calls while flying at night. Known as nocturnal flight calls, these sounds are thought to help birds stay in contact and communicate as they travel through the darkness.
They’re often brief, high-pitched sounds and can be easy to miss, particularly with insects, traffic, and other nighttime noise competing for your attention.
But once you realize birds may be calling from overhead, a seemingly quiet night can become much more interesting.
You may not see the migration.
You may be able to hear it.
Rose-breasted Grosbeak: Rose-breasted Grosbeaks migrate primarily at night, heading south from their breeding grounds in late summer and fall. Some make a remarkable nonstop flight across the Gulf of Mexico, while others follow overland routes around the Gulf on their way to wintering grounds in Central and northern South America.
One Easy Thing You Can Do to Help
There’s a downside to all this nighttime travel.
Artificial light can attract and disorient migrating birds, sometimes pulling them toward buildings and increasing their risk of collisions. The U.S. Fish and Wildlife Service recommends reducing unnecessary nighttime lighting, particularly during spring and fall migration.
The solution at home can be remarkably simple.
Turn off lights you don’t need. Close blinds or curtains when indoor lights are on at night. Consider timers or motion sensors for outdoor lighting.
Reducing unnecessary light doesn’t just make the night sky better for us.
It makes the journey a little safer for the travelers passing above.
Your Backyard Doesn’t End at the Fence
That’s perhaps the most remarkable thing about fall migration.
We spend so much time at IntoBirds watching what’s happening at the feeder, in the trees, and across the backyard that it’s easy to forget about the enormous aerial highway above it.
Tonight, when the sun goes down, step outside for a few minutes.
Look up.
Listen.
The sky may appear empty.
But somewhere in that darkness, a tiny warbler weighing only a fraction of an ounce could be heading south. A thrush may be calling as it passes overhead. Thousands of other birds may be following the same ancient seasonal pull toward places hundreds or even thousands of miles away.
And tomorrow morning, one of those travelers just might be sitting in a tree in your backyard.
That’s the magic of migration. Sometimes the best birding begins while we’re sleeping.
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