Your turn
Each lab is one part of the video. Change things and see what happens.
Each lab isolates one part of migration. Maps and numbers are simplified to show the idea.
Tap, drag and slide. See what happens!
Move the month slider. When is the hummingbird in the garden, and when is it in the tropics?
Scrub through the year: breeding season in the north, winter in the tropics, and the two migrations between.
Slide the months. When is the hummingbird in the garden?
Pick a season, then drag the hummingbird to a place where it can eat.
Switch seasons and move the bird. Where can a nectar and insect eater survive?
Pick winter or summer. Drag the bird to the flowers.
Set the hummingbird's weight, then send it across the Gulf of Mexico. Is there enough fat?
At ~480 km per gram of fat (with helpful winds), how much fat does an 800 km crossing need?
Make the bird fat or thin. Then send it over the sea!
Simplified: real range depends on wind and weather.
Drag the still point of the night sky. Which way does the young bird decide is south?
Choose the point the young bird's sky turns around, as Emlen did. The bird learns that point as north and heads away from it.
Drag the glowing star. Which way does the bird fly?
Add clouds, a confusing magnet or city lights, then fly the bunting home. Watch its fat!
Remove cues one at a time. With only one compass it still gets there; with none, or with lights, it wastes fuel.
A toy model: real birds combine many more cues.
Drag the last goose. Where does it work least?
Place the follower in the wingtip upwash of the bird ahead, then directly behind it, in the downwash.
Drag the last goose into the V. Where is flying easy?
Effort numbers are illustrative; studies estimate a V saves roughly 12–20%.
Every summer, a tiny hummingbird visits the garden. It is as light as a penny. In autumn it flies away, far to the south, where flowers bloom all winter. In spring it comes back.
Birds do not fly away just because it is cold. In winter, the flowers and bugs up north are gone. So birds fly to places where there is still food. In spring, the north is full of food again, and the birds come back to have their babies.
Before a long trip, the hummingbird eats and eats. It gets twice as heavy! The extra fat is its fuel. It can fly over a whole sea without stopping.
In the day, birds use the Sun. At night, many birds look at the stars. The sky turns slowly around one still star, and young birds learn to fly away from it. Birds can also feel a hidden compass inside the Earth.
Geese fly in a V shape. Each goose gets a little lift from the one in front, and they take turns leading.
Birds migrate by fuelling up on fat, then flying between a summer home where they raise chicks and a winter home where there's food. To find the way, they use several compasses at once: the Sun by day, the slowly turning night sky, Earth's magnetic field, and landmarks like coastlines, rivers and mountains.1 Many songbirds do most of their travelling at night.6
It's not a rare trick. Of the more than 650 bird species that nest in North America, more than half migrate, and about 350 of them are long-distance migrants, many reaching Central or South America.1
If you have a feeder in the eastern US or Canada, your summer visitor is almost certainly a ruby-throated hummingbird, the only hummingbird that nests east of the Mississippi River.2 In autumn nearly all of them leave the United States and Canada and spend the winter from Mexico to Costa Rica or Panama.4
Some take a daring shortcut: they cross the Gulf of Mexico in one non-stop flight of about 500 miles (800 km). Many others go the long way round, following the Texas coast where they can stop to eat.24 Scientists estimate the whole trip south takes two to three weeks.3
| Hummingbird | Summer home | Winter home |
|---|---|---|
| Ruby-throated | Eastern US and southern Canada | Mexico to Costa Rica/Panama; a few in Florida |
| Rufous | Pacific Northwest, western Canada, Alaska | Mexico (up to about 3,900 miles from Alaska) |
| Violet-crowned, Rivoli's | Southwest US | Short hop into Mexico |
| Anna's | West Coast, up to British Columbia and Alaska | Stays put all year |
Sources for the table: Audubon.34 And no, hummingbirds don't hitch rides on the backs of geese. That's an old myth.3 Banded hummingbirds have been found in the same yards year after year, so the one at your feeder next spring may be an old friend.3
Mostly for food, and to find good places to nest.15 In a northern winter, flowers die back and flying insects vanish, so birds that eat nectar and insects must go where food still exists. In spring, the north explodes with insects, fresh plants and long days, which is perfect for feeding chicks. Birds that migrate north to breed raise more young on average (4–6) than their relatives that stay in the tropics (2–3).1
Cold alone isn't the problem. Many species, including hummingbirds, can survive freezing temperatures as long as they have enough food.1 Hummingbirds can even drop into torpor, a short-term hibernation-like state, on cold nights to save their fat.2 Birds that can find food all winter, like chickadees picking insect eggs from bark, simply stay.1
The main trigger is changing day length. Shorter autumn days (or longer spring days) change a bird's hormones. Temperature, food supply and genes play a part too.1 Caged migratory birds get restless each spring and autumn, fluttering toward one side of the cage. Scientists call this Zugunruhe, German for "migratory restlessness".1
They fuel up first, and for a bird, fuel means fat. A ruby-throated hummingbird normally weighs about 3 grams, roughly a penny. Before migrating it can double its body mass with fat from nectar and insects. With helpful winds it can travel up to 300 miles (about 480 km) on a single gram of fat.2 Stored fat is what makes the Gulf crossing possible; many others follow the coast, where they can stop to feed.
Along the way, most migrants rely on stopover sites to rest and refuel. Along Nebraska's Platte River, more than 500,000 sandhill cranes gather each spring.5
Scientists are still working out the details, partly because birds combine many senses at once.1 Here are the compasses we know about:
Most amazing of all, many first-year birds make their first migration alone, and still find a winter home they've never seen.1
Yes. Most migratory birds fly at night, including sparrows, warblers, orioles, thrushes and ducks. The night gives them the stars to steer by, fewer hunting hawks and falcons, calmer air, and cooler temperatures so they don't overheat.6 On busy autumn nights, forecasters at BirdCast use the US weather-radar network to predict how many birds will be in the air. One forecast predicted 378 million birds in a single night.131
A major night-time danger is artificial light. Bright city lights draw birds off course. They circle lit buildings, burn precious fat and can crash into glass.56 In the US, nearly a billion birds hit glass every year.5 Turning off outdoor lights during migration season is one of the easiest ways to help.1
To save energy. A flapping wing leaves a swirl of air behind its tip, and just outside that swirl the air is rising (upwash). By flying behind and to one side of the bird ahead, each bird gets a little free lift. A 2014 study of northern bald ibises found that birds in a V sit in exactly the positions theory predicts, and even time their wingbeats to catch the upwash. Directly behind another bird is the downwash zone, and there the birds flapped out of step, which may help them cope with the sinking air.10 Canada geese take turns leading.12
It depends where they nest. Canada geese from the far north fly a long way south, while geese nesting in southern Canada and the US move short distances or not at all. Many now stay all year on lawns, golf courses and parks. In winter, geese can stay in surprisingly cold places as long as there's open water and food, and they tend to return to the same stopover and wintering spots every year.12
Most small birds take shelter: deep inside thick hedges and evergreens, on the side of a tree away from the wind, or in tree holes (woodpeckers, bluebirds and chickadees).14 Wet feathers stop trapping warm air, so small birds, which lose heat fastest, often hide before the rain starts. Ducks, geese and other waterbirds have well-waterproofed feathers and often carry on as normal.17
The Arctic tern. It nests in the Arctic and spends the northern winter in the Antarctic summer. Tiny trackers showed that some terns fly more than 80,000 km in a year, even though they weigh less than 125 grams.11 For its size, the rufous hummingbird's trip, about 3,900 miles from Alaska to Mexico, is one of the longest of any bird: over 78 million body lengths.3
Birds aren't the only long-distance travellers. European eels swim thousands of kilometres across the Atlantic to breed (see how eels reproduce), and for another bird whose life is shaped by what it eats, see why flamingos are pink.
Turn off or dim outdoor lights at night during spring and autumn migration, make windows visible to birds with patterns spaced about 2 inches apart on the outside of the glass, and plant native flowers and shrubs.5 It matters: a 2019 study estimated that bird numbers in the US and Canada fell by 29%, almost 3 billion birds, between 1970 and 2017.15
Birds eat a lot and store fat as fuel, then fly between their summer and winter homes. They find the way using several compasses at once: the Sun by day, the turning night sky, Earth's magnetic field, and landmarks like coastlines and rivers. Many fly at night.
Ruby-throated hummingbirds, the only hummingbird that nests east of the Mississippi, spend the winter from Mexico to Costa Rica and Panama. Rufous hummingbirds fly from as far north as Alaska to Mexico. Anna's hummingbirds are the exception: they stay all year along the West Coast.
Partly by instinct, which sets the direction and timing, and partly by learning. Young songbirds learn which way is south by watching the night sky turn around the North Star, and they can also sense Earth's magnetic field. With experience they add landmarks, and many return to the same places year after year.
They fuel up first. A ruby-throated hummingbird can double its weight in fat before crossing the Gulf of Mexico, and with helpful winds one gram of fat can carry it up to about 480 kilometres (300 miles). Flying at night in cool, calm air also saves energy.
Often, yes. Banded hummingbirds have turned up in the same yards year after year, and Canada geese tend to use the same stopover and wintering areas.
Mainly to find food, not to escape the cold. Many birds, even hummingbirds, can survive freezing temperatures if there's enough to eat. In a northern winter, flowers and insects vanish, so insect- and nectar-eating birds head to the tropics, then come back in spring when food is plentiful.
Most small land birds shelter before and during rain: deep inside thick hedges and trees, on the side of a trunk away from the wind, or in tree holes. Ducks, geese and other waterbirds have well-waterproofed feathers, so they often carry on as normal.