UNITED STATES (WASHINGTON POST) - It's migratory bird season in much of the United States, when about five billion birds enter the country from the tropics and remind us during their "dawn chorus" that they were the ones who invented the tweet.

The seasonal migration of so many birds is an ecological puzzle. There are roughly 10,600 known species of birds, and most don't migrate. But about 15 per cent of bird species go on these exhausting, heroic journeys, commuting thousands of miles between their summer breeding grounds and the places they spend the winter.

There's no obvious explanation for why some bird species are migratory and others are not. Big birds, little birds, loud birds, quiet birds - the migrants come in all shapes and sizes and habits. Even within a migratory species, behaviour can vary wildly.

So scientists are still trying to find the signal in all that noise - and now they may be homing in on one. A study published on Monday (May 7) in the journal Nature Ecology & Evolution presents a model that explains, in a very big-picture way, the global distribution of birds.

The researchers say the reason for migration is surprisingly simple: It's all about energy efficiency. Birds are saving energy by flying thousands of miles.

More precisely, the energy cost to a bird of flying long distances is balanced out by the energy savings of being in a place where, in summer, there are lots of mosquitoes, flies, insect larvae and other avian delicacies, and there is relatively little competition for food.

The focus on energy acquisition and energy expenditure explains not only individual bird behaviour - say, why one warbler chooses to fly from the Yucatan to Upstate New York - but also the geographical distribution of all birds collectively, according to Marius Somveille, lead author of the new report.

In the model, migratory behaviour emerges as if driven by a global mechanism to redistribute birds more efficiently. It's not just animal instinct honed by evolution; it's the biosphere arranging birds in the most logical fashion.

"There's just one rule and one mathematical model that explains the whole thing," said Somveille, who did the research at Oxford University and is now at Yale.

"I think it's a great study," said Andrew Farnsworth, a Cornell University migration biologist who was not involved with the new study.

"It's the best evidence we have so far that energy - and vegetation translated into energy - is really the guiding principle behind the patterns we see."

There's obviously a common-sense element to this: Birds seek food. Every ornithologist knows that migratory birds tend to track the "green wave" as spring growth moves north and are, in effect, going where the energy is.

But the new research says that this follow-the-energy model can explain bird distribution on a global scale (with some exceptions, such as in the Andes and the Himalayas), and that the same concept could be deployed to explain why other animals, such as fish and whales, migrate the way they do.

This could also help scientists understand the implications of climate change on the future distribution of birds and other species.

In preparing the study, Dr Somveille created a simulated planet in which birds appear first in the tropics and gradually fill it up. The study uses vegetation as a proxy for the energy supply in the environment.

For a bird, the tropics have obvious virtues, including the abundance of flora and fauna. There's no need for the bird to devote much energy to temperature regulation.

But then comes the tragedy of the commons: Everyone wants to live in the tropics. Billions of birds are crammed together, competing for the same food.

Meanwhile, the planet moves around the sun, winter turns to spring, the snow melts in the temperate zones. Viewed from space, there's all this enticing, suddenly efflorescent terrain far from the tropics.

"The tropics are so crowded, that at some point some species find it a better strategy to migrate to a place with a surplus of resources," Dr Somveille said.

Natural selection works its magic: Birds that migrate and exploit the abundant resources of the temperate zones can produce many thriving, fertile offspring.

The migratory instinct is heritable, the urge to travel switched on by hormones triggered by changes in day length.

And here they come: wood thrushes, chestnut-sided warblers, indigo buntings, Baltimore orioles, rose-breasted grosbeaks, tree swallows, broad-winged hawks. And so on.

What the Somveille study doesn't do is explain why some species become migratory and most do not. That remains an ornithological mystery.

Most migratory birds fly at night. Some come from Mexico and Central America, crossing the Gulf of Mexico in a single night, arriving exhausted on the barrier islands and coastal swamps of the US Gulf Coast.

Many don't make it.

Recent research has described "shark bellies filled with songbirds during springtime," said Dr Peter Marra, head of the Smithsonian Migratory Bird Center.

Bird conservationists this time of year worry about migratory birds colliding with buildings at night after being attracted to, or confused by, artificial light.

They also worry about habitat destruction - the loss of places where birds normally stop to rest and refuel. Another major menace to birds: Cats. About two billion birds a year are killed by cats in the US, according to Dr Amanda Rodewald, a professor of ornithology at Cornell University.

"For birds that are migrating, they really need to do two things. They need to stay alive - not get eaten by predators, not fly into buildings, communication towers. They also need to find habitats that are suitable for them to gain weight," Dr Rodewald said.

"Fat - the energy in fat - is the currency in migration."

The instinct to migrate can be overwhelmed by changes in the environment.

The white stork - known to all students of science as the bird that delivers babies - normally commutes seasonally between Africa and Europe, but many members camp out permanently at landfills in Spain and Portugal, their migratory instinct switched off by the availability of Early Bird Specials every day of the year.

"Migration is a very flexible trait," Dr Marra said.

Which is why those migratory Canada geese who leave goose poop all over golf courses refuse to leave. They're "migratory" with an asterisk.