Animals

How Do Birds Know When to Migrate?

The mechanism that tells birds to return in spring is still a mystery, but signs suggest it's triggered by weather and its effect on food supply.

If you live in a northern state or Canadian province that's been buried under snow recently, you may be eagerly awaiting the first sight of an American robin digging a worm out of your front yard as the sign that spring is truly here.

While not all robins migrate, in regions with cold winters, many numbers of the familiar avian species migrate to places such as sunny Florida and Texas for a few months and then head back north when spring returns to select a breeding territory and mate. As robin expert Roland H. Wauer notes, "The spring arrival of the American robin provides many northern dwellers with an emotional lift."

But you may wonder: Why do robins and other birds come back? And how do they know when to make the trip?

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The timing of bird migrations is one of the most intriguing phenomena in nature, and scientists are still working to solve its mysteries.

According to Ian Newton, author of "The Migration Ecology of Birds," many long-distance avian migrants are remarkably regular in their departure and arrival dates, and that's a crucial part of their continued survival. Regularity ensures that individuals arrive in nesting areas just as environmental conditions become suitable for breeding, and then leave before they change.

But birds seem to adjust to variations in weather that occur from year to year, which suggests that their migratory instincts are triggered by some sort of external stimuli, he writes.

Birds such as robins aren't that much physically affected by plunging temperatures, so the key issue for migration seems to be food availability rather than comfort. When the supply of insects or plant food becomes depleted in the north, they'll head southward to someplace where the food is more plentiful. As that supply becomes depleted by consumption and changing weather conditions in their winter home, that seems to trigger the urge to head back the warming north, where the insects and plants are starting to become available again.

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While we know what conditions attract them, scientists are still trying to figure out the mechanism that actually tells birds it's time to take off. According to All About Birds, a site maintained by Cornell University's Lab of Ornithology, one hypothesis is that birds have some sort of "undiscovered interface" - basically, a sort of biological WiFi connection - that enables them to sense distant temperature and weather conditions.

That sensing ability might explain why a shifting climate can alter migration patterns and the timing of birds' return trip, as a 2013 study by University of East Anglia researchers reported. The scientists found that over a 20-year period, Icelandic black-tailed godwits advanced their spring arrival date by two weeks, as temperatures warmed.

Many American robins migrate from the north to sunny, warm winter havens, and then return in the spring.

Birds in flight often arrange themselves in aerodynamically optimum positions, according to a new paper in the

journal Nature

that helps to explain how birds fly in such impressive formations. Lead researcher Steven Portugal and his colleagues focused their study on northern bald ibises, but many bird species also exhibit the amazing flight behavior. Portugal, a University of London Royal Veterinary College researcher, told Discovery News that birds could be using three things to achieve their flying precision: "(1) vision – watching the bird in flight to get all the information they need, (2) feathers – sensing the changes in pressure, wind etc. through their flight feathers, and (3) positive feedback – i.e. they just fly around and when it feels easier/better they stay in that position."

The researchers determined that birds try to find "good air," meaning airflows (not just wind, but even the air created by other flapping wings) that minimize their energy expenditure and help them to get where they plan to go. Conversely, birds avoid regions of "bad air" that could work against them.

Many birds fly in distinctive V-formations. Portugal said, "The intricate mechanisms involved in V formation flight indicate remarkable awareness and ability of birds to respond to the wing path of nearby flock-mates. Birds in V formation seem to have developed complex phasing strategies to cope with the dynamic wakes (turbulent air) produced by flapping wings."

Military planes sometimes fly in what is known as an "echelon formation," which mirrors nearly the exact same flight formation of many birds. This particular bird version is a variation of the "V," only with a rounded edge.

The U.S. Navy's famous flight demonstration squadron The Blue Angels often flies in a trademark "diamond formation" once popularized by fighter-bomber pilots. In it, the pilots maintain an 18-inch wing tip to canopy separation. Birds can fly even more tightly together.

The term "murmuration" refers to a flock of starlings. These birds can create dramatic patterns in the sky, such as this one over marshlands near Tønder, Denmark. Other small birds, such as sandpipers, may also create what look to be dazzling aerial ballets in the sky as they fly en masse.

Even birds flying very close to land can do so in remarkable unison. Here, a formation was photographed as the birds flew over the beach at Camperduin in the Netherlands.

The classic "V" formation has all sorts of variations. In this case, three separate -- yet united -- groups create an arrow-like effect in the sky.

Alfred Hitchcock's classic horror film "The Birds" included many scenes where numerous birds blanketed the sky. Up close, these starlings look small and harmless but, as a huge murmuration, their power becomes evident.

From the earliest planes to those in design today, aircraft have been modeled after birds. It's no wonder. Every inch of this sleek northern bald ibis, snapped while flying over Tuscany, adds to the bird's flying prowess. Its 53-inch wingspan and powerful, synchronized wing beats must have captivated people in the ancient world too, since ancient Egyptians and other early cultures featured the birds prominently in their artwork and legends.