How radar can be used to track bird migration

Flock shape, speed and structure are key to determining whether radar readings are of birds, bats, insects or weather conditions.

By Marta Hill edited by Andrea Thompson

Currently, hundreds of millions of birds make their annual journey to settle in their northern breeding grounds. As they travel hundreds or even thousands of miles, they stop every now and then to refuel, delighting birders as they go. However, the majority of their movements are at night; This is one of the reasons why experts say turn off unnecessary lights during migration season. So how can bird enthusiasts follow these migrations? It turns out that the same radar that tracks storms has an important role to play.

“If you turned on the Weather Channel or the local news and the meteorologist pointed out, ‘Oh, it’s going to rain in this area…’ those are the same radars we use as ecologists to quantify birds moving through the atmosphere,” says Kyle Horton, an ecologist at Purdue University who studies migratory birds and is a member of the association. BirdDistributiona collaboration that tracks bird migration.

Horton’s lab and the BirdCast collaboration use radar data to study bird migration patterns and generate a number of bird tracking maps which give birders an overview of the number of birds on the move and offer bird watching forecast for the next few days.

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Radar systems send out radio waves, then record any energy reflected back to their receivers. Information about the time it takes for the signal to return to the receiver is used to calculate the distance between an object and the radar tower. Just as radar beams reflect off water droplets during brewing storms, they reflect off migratory birds.

When radar aeroecologists like Horton first look at radar data, they include everything radar can detect in the atmosphere: birds, storms, insects, debris and smoke, among others. The challenge then becomes isolating specific data. “We always like to say, ‘We just do the opposite of what meteorologists do.’ They remove the birds to keep the rain flowing. We take out the rain to maintain the birds,” says Horton.

The signatures of birds and rain on radar readings are different enough to be relatively easy to separate, Horton says. Birds tend to migrate along a north-south axis, while storms generally move from west to east, for example. Storms are also highly structured and denser than flocks of birds. “The rain is really constant. It’s even. The birds are flapping their wings. They’re oriented in strange ways. There are birds of different sizes and so they’re filtered out very quickly,” says Horon.

One difficulty in isolating bird migration from radar data comes in the form of other flying animals: bats. The trick here, Horton says, is to look at the shape the herd is taking. When bats emerge from a cave, they form a donut-shaped pattern as they spread out in search of food. Certain types of birds also exhibit this behavior, but additional information can be used to determine whether birds or bats are on the radar. For example, bats also take off from a cave with a fixed location, while birds take off from a wide variety of locations.

Insects also inhabit the lower atmosphere and should be removed from the datasets, but importantly, birds can fly much faster than insects. “Entomologists wouldn’t like this, but we call insects ‘the detritus of the atmosphere,'” says Horton, because they generally move with the wind, “whereas birds can fly faster than the speed of the wind.”

Once bats, insects and weather are taken into account, the processed radar data can be transformed into cards this gives an idea of ​​how many birds are on the move at any given time – up to around 400 million birds during peak migration.

What bird migration forecasts can’t tell us, however, is which birds are flying or where they hang out during the day.

“A forecast can be one of two things. It can be birds arriving in your area, or all the birds you were looking forward to seeing the day before leaving your area,” says Horton. “We don’t have a perfect science of it, and I think that’s what makes birding fun.”