The New Science of Bird Migration

Every fall our eyes are drawn skyward by the honking of migrating geese flying in their characteristic V-formation. Yet beyond what we can see and hear – and mostly under the cover of night – millions of birds are passing overhead this time of year.

With winter pressing closer and food supplies dwindling on their breeding grounds in the Arctic north, over 350 species of birds in North America are migrating south toward their winter grounds. Some will travel short distances from their summer homes; others will fly thousands of miles to overwinter in the southern United States and in Central and South America.

Until recently, researchers have had an incomplete picture of bird migration, and for good reason: most migrators fly at night when they are not readily observed; many cover thousands of miles requiring pursuit across long distances; some birds must be captured – and then recaptured – to attach and retrieve tracking devices.

But with enhancements in technology and the help of community and citizen scientists, investigators are discovering exciting new details about bird migration. Here’s a sample of what they’re learning.

• To support their long flights, birds must increase their fat reserves and bulk up their flight muscles. So they gorge on food, sometimes even doubling their size. Simultaneously, their metabolism increases while, in some species, their digestive and reproductive organs shrink to off-load unnecessary bulk. While they should be on the verge of a metabolic catastrophe, they are in prime condition.
• Migrating birds use a combination of navigational strategies. One is to use celestial cues like the Sun, the Moon and the stars to help them plot their course. Other birds follow terrestrial landmarks like mountains and rivers to guide their journey. But when visual mileposts are hidden by conditions like fog or darkness, it appears that some migrators navigate with the help of the Earth’s magnetic field. Using this super-vision, they build a map in their brains that helps them chart their direction.
• Rest is critical during migration, so many birds stop along their route to recharge and refuel. But how do non-stop flyers rest? While the research is ongoing, scientists suspect that long-distance migrators have a novel way to sleep. It appears they shut down one hemisphere of the brain and close the corresponding eye affording them brief episodes of sleep. They nap in flight.
• Early studies of migratory birds focused largely on their winter and breeding grounds where the birds could be easily observed. But today researchers are studying the connecting piece – the migratory route. Already, the findings are impressive, but some of the news is sobering: collisions with tall structures like skyscrapers and communications towers; navigation confusion caused by urban lights; degradation of trusted stopover and breeding sites; even predation by domestic cats – all these threats are contributing to the evidence that migrating birds are in decline.
• Thanks to technology, scholars are learning stunning details about migration that will lead to new conservation measures. Here are some of the tools they are utilizing.
  • Advanced telemetry, including a global network of receiver stations, record the signals of birds outfitted with tiny transmitters. These devices can pinpoint a bird’s location; record environmental factors like latitude and longitude or sunlight and temperature; some even capture details like flight speed and wing flaps.
  • Acoustic recordings detected by land-based microphones help identify the calls of nighttime migrators. This technology provides important new details about the diversity of species migrating over a certain location.
  • In research labs, scientists are analyzing birds’ feathers, blood and feces to determine where birds are foraging and what migrating population they belong to. These details help conservationists identify and protect feeding grounds along the migratory route.
  • NEXRAD radar, a national network of doppler weather radars, supplies data without the need to capture and tag any birds. Instead, the radar creates a visual signature of the movement, density and speed of large flocks of flying birds. Some radars can even detect birds at the moment they leave the ground providing researchers with the location of resting and feeding sites.
  • Community/citizen scientists and bird enthusiasts are adding to a growing data set by reporting sightings to investigators. This on-the-ground detail is helping authorities track the location and distribution of entire populations of bird species as they move across the planet.

Due to the work of researchers and community volunteers, the science of bird migration is enjoying a resurgence that amazes even the casual birdwatcher. Flying overhead is one of nature’s most colossal spectacles: millions of birds, some weighing less than a quarter, engaged in an ancient ritual that we’re only beginning to understand. Remarkably, they will repeat their journeys when spring urges them to return to their ancestral breeding grounds to raise another generation of migrating birds. And once again, our eyes will be drawn skyward—with renewed wonder.

Thanks to Dr. Michael Ward, a professor in the Department of Natural Resources and Environmental Sciences and ornithologist with the Illinois Natural History Survey at the University of Illinois at Urbana-Champaign. As a specialist in the ecology and behavior of birds, his expertise was valuable in writing this article.

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Carla Rich Montez is an Illinois Master Naturalist volunteering as a writer for Outdoor Illinois Journal. While she was writing this article in August and September, over 60 million migrating birds crossed over her Illinois county.