Full moon casts light on bird migration

[Donna]

The News Journal • November 1, 2009

Over the next few nights, David LaPuma and dozens of other birders will point telescopes, spotting scopes and binoculars toward the sky to count the birds that fly past.
One of the tools they'll use is one that's not in their bag of magnifiers: The moon.
Thanks to the full circle of white sitting in the sky, they'll be able to note the direction of travel and estimate a bird's size in comparison to the lunar crater Tycho. And when the evening of moonwatching is complete, they will submit the data to the Fall Migration Moonwatch Project 2009.
It's not always easy. The birds often whirl past the moon, making it almost impossible to tell what kind they are, said LaPuma, a researcher in the ecology and evolution program at Rutgers University Cook College in New Jersey. With 60 birds crossing the moon in an hour, an experienced bird watcher may see one and say, "Oh, that looks like a duck," he said.
LaPuma hopes that moonwatching and other tools can help birders sift through the feathered fliers and gain a better understanding of when, how and why birds migrate.
This information will help protect declining species, find patterns or changes in bird movements, and identify and protect critical habitats, LaPuma said.
Some birds get to one place and stay there. But for most species, migration is a way of life and survival.
And there's lots of things researchers already know about migration. They know a warmer spot isn't always the motivation for a bird on the move -- some are following the food.
"The birds are going to be stopping over where the food resources are," said Anthony Gonzon, who is compiling Delaware bird data for the state's Breeding Bird Atlas. "This time of year, the abundance of insects is declining."
A good stopover place is often an area that has fruits, nuts, seeds and berries, Gonzon said.
Contrary to popular belief, birds aren't always like humans in their resting patterns.
In migration, hundreds of species move at night.
East of the Rocky Mountains, an estimated 300 to 400 species of birds can and do migrate at night, said Andrew Farnsworth, a scientist in the evolutionary biology program at the Cornell Lab of Ornithology. Many of the night fliers are songbirds -- warblers and sparrows -- but owls and even waterfowl will migrate at night, he said.
Why? Scientists believe the atmosphere is smoother at night and birds can travel farther and faster with less effort. They may also be less vulnerable to predators.
Those who study migration believe there likely are three primary strategies that birds use: piloting, in which birds use familiar landmarks to move from place to place much like our daily commute; orientation, when birds use the sun, stars and Earth's magnetic field to navigate; and true navigation, in which birds use a mechanism to correct their course in windy conditions or bad weather.
Some birds travel great distances -- like the red knot in its spring migration from South America to Delaware Bay and then on to Arctic breeding grounds. Others, like neotropical migrants, travel short distances of 150 to 200 miles every night and then stop in critical habitats to refuel.
"The birds are going to be stopping where the food resources are," Gonzon said.
Some species follow ridge lines and some travel along coast lines, he said.
And sometimes, as they start their migration, large numbers of birds will build up on the edge of weather fronts and use the upper atmospheric winds to their advantage, he said.
These birds aren't wimps. They are often hundreds of feet above the ground. Typically, they fly just after dusk and begin to fall out an hour or two before dawn, Gonzon said.
Radar, acoustic devices
Watching the moon and counting the birds isn't the only tool researchers use.
Some scientists use radar to track bird movement. That gives a picture of the numbers of birds that are migrating.
But to find out what is moving, researchers use acoustic devices and computers to pick up flight calls of birds in migration.
Still others rely on teams of volunteers to ground-truth the high-tech data sets by sending teams out the following morning to see what is feeding and resting in migratory hot spots.
On Thursday night, for instance, LaPuma could look at a radar image and see that a big flight of birds was moving through the region. Scientists can tell the difference between birds and thunderstorms, for instance, or birds and dust, or birds and insects.
Birds typically fly 10 to 15 knots faster than the wind speed and they don't always travel downwind, according to experts at New Jersey Audubon
Insects, dust and rain are all wind-blown and move in the same direction and speed as the wind.
If the object in the radar moves 10 knots faster than the prevailing wind and is travelling crosswind or into a headwind, researchers know it is almost always birds.
Radar gives a broad-brush picture of migration, said Vince Elia, a research associate at New Jersey Audubon. It can help tell where groups of birds are headed, he said. But it does not tell what kind of bird.
Often in that radar image, there is a range of birds in a wide variety of sizes, he said.
That's where scientists like Andrew Farnsworth come in.
Farnsworth, a research associate in the conservation program at Cornell University Ornithological Labs, has moonwatched and used radar, but he also uses sound and recordings of migration flight calls to determine what is moving through an area.
Using computers, researchers have been able to come up with signature calls -- different from the chirps and tweets we hear in our backyard -- that many species use in migration. These sounds may help the birds stay together or space themselves in flight, he said.
Farnsworth said migration work is important because for some species it may be the only way to get a snapshot of habitat and population data.
Some birds breed in places where no one is doing surveys and these species may slip through the data collection done in the Christmas bird counts.
So for some species, a migration count may be the best way to gather data, he said.
But for other species, it is yet another tool in coming up with management and conservation strategies. The data could be especially important as rural areas become urbanized and as climate change impacts habitats, he said.
On a good night for migration, 30 to 45 minutes after the local sunset, "you see this incredible blooming in the sky," Farnsworth said of the radar images.
And then, as the birds land, the radar gives a clear picture of places where birds spend the day and what locations might be at risk, he said.
LaPuma combines radar data and other tools and produces a Web site called Woodcreeper to keep people informed about birds' migration.
The fall migration, for instance, usually starts in September and ends in November.
The spring migration is compressed and typically occurs in April and May in our area.
But in the end, LaPuma sees it all as an important tool in protecting critical habitat.
"You can predict where birds are going to be," he said. "It's fun stuff."