A method has been devised by scientists that uses radar weather maps to predict the movements of millions of birds as they fly through the skies above the continental United States
Twice every year, billions of birds wing their way across the skies above the continental United States. Between March and May, adult birds migrate north from their tropical wintering grounds in Central and South America to North America, where they breed in early summer. Autumn migration is even larger than spring migration: between August and late October, waves of adult birds and their juvenile offspring fly south from North America to their overwintering sites where it is warm and food is plentiful.
“Most of our songbirds migrate at night, and they pay close attention to the weather,” said Benjamin Van Doren, a zoology Ph.D. student at the University of Oxford who specialises in using a range of innovative tools and technological approaches to document how birds adjust migratory behavior in response to both short- and long-term environmental changes.
Mr. Van Doren and his collaborator, Kyle Horton, a Rose Postdoctoral Research Fellow at the Cornell Lab of Ornithology, decided that they would better understand migration if they followed their study subjects’ example: they too would pay close attention to the weather to learn what cues migratory birds were responding to.
Everyone has heard of weather forecasting and most of us are even aware that flocks of migrating birds show up on Doppler radar (one example), a highly sensitive tool used by meteorologists to track and forecast weather. But Mr. Van Doren and Dr. Horton took their research a step further: they wanted to know if they might actually figure out how to forecast bird migrations like meteorologists forecast the weather.
According to a new study (ref), Mr. Van Doren and Dr. Horton report that they have figured out how to reliably predict when large flocks of migratory birds will begin their migratory journeys — remarkably, sometimes as long as seven days in advance.
To do this, Mr. Van Doren and Dr. Horton collected 23 years of spring bird migration data captured by 143 weather radar stations across the United States. They measured the density of migrating birds detected by these weather maps during the same time period each night between March and May. Although it could neither detect individuals nor identify species, the radar was sensitive enough to detect the difference between a light migration (60-70 birds per cubic km) and a heavy migration (1700 birds per cubic km). Based on these data, Mr. Van Doren and Dr. Horton estimated there is an average of 200 million birds migrating through the skies above the United States, and this number swells to more than 429 million birds each night in early May during peak spring migration, and to 520 million birds during peak autumn migration during the month of September.
Next, they combined their radar data with weather conditions such as temperature, wind, and air pressure, to identify which conditions birds tended to migrate in.
“We used twelve variables to model the distribution of migratory birds across the continent,” Mr. Van Doren explained in a press release. “Temperature was the most important variable. Migration intensity was greatest on warm nights, probably because warm temperatures generally bring favorable winds and the emergence of leaves and insects.”
But instead of merely tracking bird movements in the past or in real time, Mr. Van Doren and Dr. Horton went a step further: they developed their model to predict bird migrations in the future. To do this, they filtered out precipitation data from the raw data and developed a machine learning tool that associated atmospheric conditions with bird movements (Figure 1).
Using those data, Mr. Van Doren and Dr. Horton constructed a model that predicts bird migrations across the continent.
“Our model converts weather forecasts into bird migration forecasts for the continental United States,” Mr. Van Doren stated. In the maps, “warm” colors correspond to greater densities of birds.
In fact, eighty percent of the observed variation in bird migration intensity was explained by their model — which is actually quite good compared to the accuracy of local weather forecasts.
A variety of bird migration maps are available on the Cornell Lab of Ornithology website, BirdCast, where they are updated at least every six hours, depending upon the map. In addition to maps featuring real-time bird movements based on current radar weather data, and estimates of nightly migratory waves of birds across the entire country, the site also predicts bird movements up to three days in advance, a feature that was added earlier this year. Mr. Van Doren, who originally started working with BirdCast in 2012, noted that “automated migration forecasting is a whole new frontier.”
“This is the most significant update since we first began using radar to study bird movements,” Dr. Horton said. “From the bird watcher’s perspective, if you know where and when migrants will be flying at night, you stand a better chance of seeing them, especially if the birds make a stopover in your area.”
But besides avid birders and ornithologists, what use will these “BirdCast” maps have for non-birders?
“The capacity to forecast where and when birds are likely to be flying is instrumental for conservation goals,” Dr. Horton explained in a press release.
Birdcasts could be very effective tools for motivating people and communities to voluntarily make minor adjustments to their behavior that could save many birds’ lives.
For example, independent of “BirdCast” data, the city of Toronto enacted a voluntary “Lights Out” program in 2009 where lights are turned out on all city-owned buildings after work and on weekends to reduce needless deaths of migrating birds. Toronto also developed common-sense bird-friendly lighting guidelines to help building owners and managers, tenants and home-owners to reduce their impact upon migratory birds (PDF) — not to mention decreasing their electricity bills.
“Our forecasts could prompt temporary shutdowns of wind turbines or large sources of light pollution along the migration route,” Mr. Van Doren pointed out. “Both actions could significantly reduce bird mortality.”
Many birds migrate at night to avoid predators, and they evolved to use the stars to navigate. Thus, they are attracted to — and disoriented by — artificial lighting, which lures thousands to their deaths through collisions with buildings and windows, wind turbines, power lines and other human-built structures or through sheer exhaustion. In fact, collisions with man-made structures are the second leading cause of bird deaths in North America after cats.
“There’s almost this magnetic pull of birds to these lights,” Dr. Horton pointed out.
“Radars have been illuminating the movement of birds for nearly 75 years — there are still integral discoveries to be made,” Dr. Horton said. “With migration coming into full swing, we’re excited to deliver autumn forecasts for the first time.”
Benjamin M. Van Doren and Kyle G. Horton (2018). A continental system for forecasting bird migration, Science, 361(6407):1115-1118 | doi:10.1126/science.aat7526
Radar Weather Maps Can Forecast Bird Migrations And Aid Conservation | @GrrlScientist
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