Snow geese glide past a setting sun at the Quivira National Wildlife Refuge near Great Bend, Kan. (Travis Heying/Wichita Eagle/Associated Press)Bird migration looks like a bad idea at first glance — all that energy needed to fly thousands of kilometres, all those predators along the way and the promise of doing it all over again just a few months later.

But of course Mother Nature knows exactly what she is doing. If you've ever wondered, as we did, what all that back and forth across the sky is about, read on.

Why migrate in the first place?

Two words: food and babies. It turns out the longer days of the northern hemisphere's summer mean a bumper crop of yummy bugs, which in turn means more baby birds.

"Even though migration is quite an investment and quite risky, the payoff can be pretty huge," says biologist Jody Allair of Bird Studies Canada, a non-profit conservation group.

"If you've spent any time up in a bog in Algonquin Park in early June, you'll know why. The food abundance is just out of control."

But the good times don't last forever. Starting in late summer, the bug banquet tapers off, and within weeks there's a chill in the air signaling the killing cold to come. Time to head south.

USFWS bird biologist Brian McCaffery conducts shore bird research inside the Yukon Delta National Wildlife Refuge in southwest Alaska. (U.S. Fish and Wildlife Service/Associated Press)

How do birds' know when to move on?

The trigger is changes in daylight — less of it in the fall and more of it in the spring. While it's not fully understood, scientists believe birds' hormone levels are affected by shifts in daylight hours, signaling that it's time to get going.

How do they know which way to go?

Juvenile birds that migrate with their parents of course are shown the way. But there are many instances where birds fly solo and still stay true to the route.

It would appear they sense the Earth's magnetic field and use it to navigate. They also adjust their path using the position of the stars and the sun.

"It's amazing to think of a 15-gram bird and the complexity of their brains and that it shows them how to fly from North America to Argentina," says Allair.

Do all birds migrate?

No. Downy woodpeckers, bald eagles and black-capped chickadees, to name a few, don’t generally migrate north and south. But they do sometimes move around their home range if local food sources run thin.

Some Canada geese are year-round city residents. (David Coates/Detroit News/Associated Press) Some species are more likely than others to stay put. Gray jays, or whiskey jacks as they are better known, are homebodies because they're so good at hiding food for later use. In Algonquin Park, some family groups have been known to hide goodies in 1,300 different caches over the warm months so that they're nicely provisioned come winter.

Even among species that do migrate, some groups will stay in one place year-round. Many major North American cities are home to resident Canada geese for example. They'll stick around as long as they can find open water and food, often in the form of ornamental berry plants in urban gardens.

How do birds keep up their stamina over such long distances?

They bulk up before and during their arduous journey, favouring fat-rich insects over seeds. Intriguingly, they pack on the pounds in an aerodynamic way by storing fat in places that allow air currents to slip by, such as in their lower backs.

Scientists have recently discovered that birds will also break down a significant amount of their digestive organs and muscles in order to fuel their migration.

No wonder some will drop their body temperature during the chill of the night in order to conserve calories. Researchers have shown that heavier birds drop their metabolic rates the least, while lighter birds drop the most. Some conserve a remarkable 30 per cent of their energy by becoming hypothermic.

Bar-tailed godwit (Tim Bowman/U.S. Fish and Wildlife Service) What bird holds the record for the longest known migration?

A bar-tailed godwit. It's a large orange/brown bird that favours mudflats and marshes. In March 2007, satellite transmitters tracked a bar-tailed godwit that traveled from New Zealand to the northern end of China’s Yellow Sea in a continuous nine-day flight of 10,200 km.

Why do many birds fly at night?

Birds typically begin migrating about a half hour after sunset, peaking at about two hours after sunset. By that time, the wind has died down, making for easier flying. Scientists also suspect night flights are attractive because there are fewer avian predators about.

Why do some birds fly in a V-pattern?

It's all about energy conservation. The birds in the front of the V are doing most of the work while those flying directly behind them are taking advantage of the forward guards' uplift, much like racing cyclists. But no matter how fit, no bird can sustain the front position for long and so they switch off at regular intervals.

Sources: Bird Studies Canada, Boreal Songbird Initiative

With files from John Bowman, CBC