This research effort was supported by a grant from the National Geographic Society's Committee for Research and Exploration. It was led by Martin Wikelski, a professor of ecology and evolutionary biology at Princeton University in New Jersey.
Birds, as they are for many naturalists, are Wikelski's first love. To understand what the feathered critters are exposed to in the air, he learned how to hang-glide. "Then I got interested in the incredible exercises such as long-distance bird migration," he said.
This interest led Wikelski to a collaboration with Visser and other colleagues in Illinois and The Netherlands to calculate what it costs songbirds in the currency of energy to chase their optimal climate as it oscillates between the continents.
To perform this calculation, the researchers captured free-flying songbirds in southeastern Illinois, tagged them with radio transmitters, injected them with a concoction used to measure energy expenditure, and then re-released them into the wild.
When the birds took to the air in migratory flight, the researchers piled into a car and sped along back roads as far north as northern Wisconsin, tracking the birds to where they landed, which happened to often be backyard gardens of suburban homes.
After knocking on doors in the wee hours of the morning and politely explaining their intentions, the researchers would enter the yard and recapture the bird by setting up mist nets. They would then take blood samples to determine how much energy the birds expended.
The measure of energy expenditure comes from a process known as doubly labeled water. The water is made up of heavy oxygen and hydrogen isotopes that mix in with the entire body water pool.
A comparison of blood samples taken just after injection of the doubly labeled water and 24 hours later allows the researchers to calculate how much heavy oxygen was converted into carbon dioxide, and thus how much energy was expended.
"We can use this heavy isotope method to understand energy expenditure of these birds in the field," said Visser, one of the world's leading experts on the doubly labeled water technique.
As a control, the researchers also injected songbirds that did not make a migratory flight with doubly labeled water. The researchers found that while flight is as costly as predicted by the theoretical models, resting at stopover sites can also be quite expensive.
"Our studies taught us that migration is not that demanding, not more demanding than feeding large young in a nest," said Wikelski. "However, cold nights, at least during spring migration, are very expensive for songbirds."
According to their calculations, on a cold spring night, a bird spends as much energy to keep warm as does a bird in a two-and-a-half-hour flight. However, any day that a bird flies longer than two and a half hours is much more expensive than a full day at rest.
"Stopover is cheaper than a night with long migration, but once a bird has done a long stretch in flight it may stay a week or so in a site to accumulate body reserves, so that offsets the energy expense of night flights," said Visser.
Interestingly, the researchers note, knowing the cost of rest helps explain why some birds change course during their migrations when the weather turns inclement.
For example, if a bird hits cold weather on its way north, it may decide to fly south to a warmer zone where it is less expensive to store up the energy required for the duration of the migration.
According to the authors, this research has important implications for understanding songbird ecology and hence should aid in songbird conservation.
For example, said Visser, knowing that the songbirds need to eat a bunch of worms and snails to build up their fat reserves for a night of flight means that the songbird migration route must include well-spaced stopover sites that provide adequate sustenance.
"It is important to have habitat within a certain distance range," he said. "If a bird would need to fly 24 hours to reach another area it would never be able to accumulate the amount of energy required to accomplish the flight and the bird would be trapped."
In their experiments, the average duration of a songbird flight was 4.6 hours, during which a bird would cover about 165 miles (265 kilometers). Songbirds fly at night, according to Visser, to avoid predators and probably because it is the costliest part of the 24 hour period to stay warm so they might as well be flying.
"It may be cheaper for them as an overall strategy that they avoid the most expensive period of sitting," he said.
Rappole said that while this research confirms the importance of stopover sites, more research is needed to understand what kinds of stopover habitats are required for successful migration and make efforts to preserve them.
"We are a long way from understanding the kinds of energy trade offs birds are forced to make during migrationbalancing their competitive abilities and experience against stopover habitat availability, weather, and fitness pressures to reach the breeding grounds in a timely fashion," he said.
Such research efforts, he added, are the basis of the National Wildlife Refuge system, which was established to ensure quality habitat for migrating waterfowl and the designation of habitat for migrating shorebirds such as piping plovers and red knots.
"The songbird people have had less success, mainly because songbirds stop just about anywhere if they are over land when daylight comes," he said. "But the fact that they stop anywhere does not mean that they can find suitable habitat to replenish fat reserves anywhere."
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