Timing the migration

Posted: Friday, September 24, 2010

While out for a hike, I thought I heard a snippet of sound on the wind. I stopped, and there it was again, a distant honking. Scanning the sky, I finally spotted a wavering "V" of geese flying so high they looked like faint pencil scribbles. As they passed in front of a mountain, it bounced the sound of their honking back louder and steadier. They were too distant for a visual, but based on their high-pitched calls, I suspect they were Greater White-fronted geese. It was late afternoon, sunny and calm, and the birds seemed like they were on a mission to fly for hours more.

Flocks of geese and sandhill cranes flying overhead are sure signs of autumn but how do the birds know when to go? The answer involves two parts: an accurate internal "clock" triggered at certain times of year and environmental cues, such as weather.

Like a store clerk starting to turn off the lights at closing time, the changing length of daylight signals to the birds it will soon be time to leave. The shorter days trigger hormones that cause phenomenal physical changes to prepare migrating birds for the ordeal ahead. They begin eating more and storing fat that will serve as an energy source for the long journey. Long-distance migrants may double their weight. Some birds even molt before migrating.

As the light continues to change, birds grow increasingly fidgety. Even in captivity, birds that would normally migrate wake just after dark and will flutter and hop in the cage until about midnight. Called "migratory restlessness," the movements have direction to them. Experiments with songbirds show the restless movements were aimed south in the fall and north in the spring.

Once the anxiety sets in, birds pay close attention to the weather. You may see geese or other birds doing a few trial runs in flocks, preparing for a good travel window. Researchers aren't sure how birds sense the weather, but they seem to be sensitive to changes in barometric pressure. Theories suggest birds preparing to migrate may be able to feel when a pressure system with favorable winds is approaching. Radar studies of migrating birds show they wait for good wind direction, and will even change their altitude to take advantage of a tailwind.

When I lived in Montana, one crisp fall morning after a storm I spied a male western tanager crouched on the ground near a truck. He was so puffed up he looked like a bright yellow baseball with a fiery red head and sooty black wings. Catching movement underneath the vehicle, I peered under and uncovered eight other tanagers huddled there. They were so tired they barely even moved their heads to look at me.

Despite precautions, storms may blow migrating birds off-route, especially songbirds. The morning after a storm is often a good time to look for a "fallout" of birds in shrubs and trees, or on the ground. Fallouts tend to be larger in areas where birds have to cross an expanse of water since they alight on the first land they find. The refugees may seem unusually calm, so you can have a good look. Don't approach too closely though, to avoid stressing them even more. The birds are as exhausted as someone who just finished a marathon - and then had to run back. After a storm could also be a good time to search for rare birds; the winds may have blown a lightweight visitor drastically off course.

Different species of birds have different strategies for the time of day they migrate, so you may need to vary when you look for them. Hawks, eagles and other soaring birds fly during the day to take advantage of warm air rising from the ground. They don't have to flap their wings when they ride these thermals, hence they save energy. Waterfowl aren't as picky; they'll fly any time of day or night. Shorebirds and some songbirds, such as thrushes, sparrows and warblers, fly at night and refuel by day. It's the darkness that hides them from aerial predators. Additionally, the air tends to be calmer at night, so they spend less energy fighting turbulence. If you're outside when these stealthy nocturnal flyers pass overhead, listen for their soft flight calls betraying their position. Other songbirds migrate during daylight. Swallows, for example, need light to see the insects they hunt in the air.

Altitude is another factor birds strive to use to their advantage, searching for the most favorable winds. Songbirds generally fly the lowest, around 2,000 feet or less. They will move higher if the air is too turbulent, however, or if they can catch a good tailwind. Shorebirds tend to fly at higher altitudes, between 6,000 to 13,000 feet. They may also move higher if the circumstances call for it. Bar-tailed Godwits - which migrate from Alaska to New Zealand in the longest non-stop, flapping flight of any bird - have been seen at 20,000 feet. Waterfowl generally fly up to 4,000 feet, but pilots have reported Canada Geese at 9,000 feet.

Why do geese fly in "V"-shaped formations? They are using the air currents from the goose ahead, a little like bikers drafting in the Tour de France. When a bird flaps its wing, the air flowing over the wing creates a small amount of turbulence at the wing tip. By flying in line near the wing tips of the goose ahead, each bird decreases some of the turbulence off its own wings. Reducing turbulence means less drag, or resistance, which translates into using less energy. For small birds, which are already lightweight, it's not a big difference. For larger birds, such as geese or ducks, the savings are worthwhile: They may use up to 40 percent less energy.

Migration is the original ultimate race. Every trick of wind, weather, timing and physics may be just the edge a bird needs to survive. Knowing these tricks gives a wildlife spy a better chance of catching the spectacle.

• Beth Peluso is a freelance writer, illustrator, and avid birder. She enjoys spying on wildlife around Alaska.

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