Why Birds Fly Far South--Or Not so Far
Whoever coined "birdbrain" as an epithet meaning "stupid" didn't live in interior Alaska. As I write this, a fine blizzard is howling outdoors, scouring fields that hold no birds. All the cranes and geese flew south; most of the people didn't.
To be fair, avian migration isn't a thoughtful process, even if it does look like a wise choice by this time of year. And it isn't undertaken primarily to escape from the cold. If they stayed in the high latitudes through winter, most of our transient bird species would starve before they'd freeze. Scientists understood that migrating birds sought wintering grounds that were good feeding grounds, and assumed that the tropics provided especially abundant food because so many birds flew that far.
The problem with that assumption is that sometimes birds bound for the tropics fly past food-rich areas in the temperate zone. They even fly past flocks of conspecifics, birds of the same kind apparently doing perfectly well on wintering grounds hundreds of miles north of the tropics.
Eventually, someone had to look into the puzzle of why migrations end where they do. According to a recent issue of Nature, a team of Dutch ornithologists has reported on a several-year study that helps explain birds' choice of destination.
For twenty years, the Dutch have been making detailed observations on wading birds of the Wadden Sea. At the spring and autumn population peaks, when Arctic-breeding birds join the locals, this shallow inlet of the North Sea houses about two million waders. Winters there are chilly but not frigid, and the birds remaining behind when the others head south have plenty to eat.
The southbound flocks nearly all winter on the Banc d'Arguin, an expanse of tidal mudflats in Mauritania, West Africa. For the last several years, the Dutch ornithologists have also migrated south in winter to West Africa.
They found millions of birds, but very little for the birds to eat. The waders are dense on the African mudflats---there are almost four times as many per unit area than is typical for temperate-zone wintering areas. However, the creatures on which they prey form less dense populations than they do on temperate flats, and are usually much smaller. The average mass of worms taken by a wading bird on the Banc d'Arguin, for example, is 3 to 10 times lower than that of the smallest worms taken in Europe.
Abundant food couldn't be the reason some birds flew as far as the tropics---the food wasn't abundant. The logical next choice was thermoregulatory efficiency: birds could get by on less food in tropical climes because they needed fewer calories to keep warm. The Dutch team found evidence to support that hypothesis. From environmental measurements---such things as humidity, wind, temperature, sunshine intensity and duration---they estimated that one species of wading bird used only half the energy to regulate its body temperature in Africa that it needed on the Wadden Sea.
That finding fit nicely with work done by a graduate student at the University of Pennsylvania, who found that sanderlings wintering in New Jersey had twice the energy requirements of sanderlings wintering in Panama. Other bits of research support what has become the new assumption about why birds undertake the longer journey to the tropics: it takes less energy to stay alive in the tropics.
I suppose Alaskans really should have been the ones to think of that. People who cheer-fully put away a pound-plus steak for lunch while they're working at Prudhoe can be found settling for a fruit cup or a salad when they're on R & R in Honolulu. We like the tropics for lots of reasons, but quantity of food seldom heads anyone's list.
