Here’s how we trace the ancient paths of bird migration

Although it still feels like beach weather in much of North America, billions of birds have begun to take wings for one of nature’s great spectacles: autumn migration. Birds fly south from the northern United States and Canada to wintering grounds in the southern United States, the Caribbean, and Latin America, sometimes covering thousands of miles. Other birds leave temperate Eurasia for Africa, tropical Asia or Australia.

Using observational records and data collected through bird band, 20th-century ornithologists roughly mapped general migration routes and timing for most migratory species. Later, using radar at airports and weather stations, they discovered how weather and other factors affect when birds migrate and how high they fly.

Today is technological progress provide new insight into bird migration and shows that it is more complex and wonderful than scientists ever imagined. These new and constantly improving technologies are key aids in protecting migratory birds from habitat loss and other threats.


Bird tours across borders

The power of the Internet has greatly aided research into migratory birds. Using the popular eBird networkbirders all over the world can upload observations to a central database, creating a real-time record of the ebb and flow of migration. Ornithologists have also learned to use NEXRADa national network of Doppler weather radars, to visualize birds that descend on the North American continent.

Now researchers are creating a global network of receiving stations called the Motus Network, which currently 1,500 recipients in 31 countries. Each receiver constantly detects the presence of birds or other animals within a nine-mile (15-kilometer) radius, which scientists have equipped with small, lightweight radio transmitters, and shares the data online. The network will become increasingly useful for understanding bird migration as more receiver stations become active along migration routes.

Tracking of individual birds via satellite

Three new technologies are rapidly expanding what we know about bird migration. The first is satellite telemetry of bird movements. Scientists equip birds with small solar-powered transmitters, which send data about the birds’ location to a satellite and then on to a scientist’s office computer. The researcher can learn where a bird is, the route it took to get there, and how fast it is traveling.

E.g rod tailspove, a pigeon-sized shorebird, breeds in Alaska and then migrates to New Zealand. Satellite transmitters show that mines often fly non-stop from Alaska to New Zealand. Recently, a gudwit set a record for the longest direct flight by a land bird: 8,100 miles (13,000 kilometers) in 10 days, from Alaska to Australia.

Bar-tailed Godwits have the ability to correct their course if they are blown off track on their epic migration journey.

Satellite telemetry studies show how much individual birds, even those from the same breeding site, vary in their migration behaviour. Individual differences in migration behavior are likely due to differences in physical condition, learning, experience and personal preferences.

Another shorebird, the one the spin, also makes a phenomenally long journey across the ocean. Satellite telemetry has shown that some eddies travel from northwestern Canada, across the North American continent to the east coast of Canada, then set off across the Atlantic Ocean on a 3,400-mile (5,400-kilometer), six-day direct flight to the coast of Brazil. In total, they can travel 6,800 miles (11,000 kilometers).

Unfortunately, hunters kill some of these birds when they land to roost islands in the Lesser Antilles. The unfortunate fate of two satellite-tracked eddies has catalyzed a campaign to tighten the rules on shorebird hunting in the Caribbean.

Geotagging of small birds

Many birds are too small to carry a satellite transmitter. Because of the energetic effort required for migration, a unit must weigh less than 5% of a bird’s body weight, and many migrating songbirds weigh less than 0.7 ounces (20 grams).

An ingenious solution for small birds is a geolocator tag or geologist – a tiny device that simply records time, place and presence or absence of sunlight. Scientists know the time of sunrise and sunset on a given date, so they can calculate a bird’s location on that date to within about 125 miles (200 kilometers).

A painted bunting equipped with a 0.024 ounce (0.7 gram) solar geolocation data logger.
Jeffrey F. Kelly, CC BY-ND

Birds carrying geologists must be recaptured to download the data. This means that the bird must survive a move around and return to the same place where it was first caught and tagged. Amazingly, many geologist-tagged small birds do.

Geologists have shown that Blackpoll singers – small songbirds that breed in the boreal forests of North America – fly long distances across the Atlantic in autumn, en route to the Amazon basin. Birds that breed in eastern North America migrate across the Atlantic into Maritime Canada or the northeastern United States and make a 60-hour, nonstop, 1,500-mile (2,500-kilometer) flight to the Greater Antilles. There they rest and recover, then continue across the Caribbean to South America.

Black pollocks that breed in Alaska fly across the North American continent before leaving the coast on the Atlantic coast and flying to South America. In total, they travel 6,600 miles (10,700 kilometers) over 60 days.

Even more amazingly, geologists show that another small songbird, the northern wheat, migrating from North America to sub-Saharan Africa. Wheatears that breed in Alaska fly 9,100 miles (14,600 kilometers) across Asia to East Africa, taking three months to do so. Those that breed in eastern Canada travel 4,600 miles (7,400 kilometers) across the Atlantic to Europe and then on to West Africa – including a 2,100-mile (3,400-kilometer), four-day nonstop overwater flight.

Registration of night migration calls of birds

Two hours after sunset in the fall, I like to sit outside and listen to birds chirping overhead. Most birds migrate at night, and many give a species-specific “chit”, “zeep” or other call note during flight. The calls can serve to keep migrating flocks together, including different species on their way to the same destination.

Ornithologists use automated passive acoustic recording to study these nocturnal calls and identify the species or group of related species making each sound. The technology is a sky-pointing microphone connected to a computer that continuously records the sound stream and is aided by sound recognition software. Sometimes it reveals migrants overhead that are rarely seen on the ground.

Scientists use infrared cameras and birds’ nocturnal migration calls to assess the risks birds face by colliding with buildings.

Nick Kachala, an honors student in my lab, set up recording units on three university properties in the fall of 2021. One of the most common recorded migrants was grey-cheeked thrush, a shy bird of the northern boreal forest, rarely seen in the northeastern United States during fall migration. He also discovered dick chisela grassland bird that I have never seen in our area.

Many bird watchers are now building DIY backyard recording devices to identify the birds that fly over their home during migration.

Conservation of migratory birds

Radar monitoring indicates that the number of North American migratory birds decreased by 14% between 2007 and 2017. There are probably several reasons, but habitat loss is probably the main culprit.

Satellite telemetry and geologists show that there are special stopovers along migration routes where migrants rest and refuel, such as Texas Gulf Coast, Florida Panhandle and Mexico’s Yucatan Peninsula. Conservation experts widely agree that protecting migratory birds is critical preserve these places.

Effective conservation measures require knowing where and how birds migrate and what dangers they face during migration. Ornithologists using these new technologies are learning things that will help stop and reverse the global decline in migratory birds.

The conversation

Tom Longprofessor of biology, Clarkson University

This article is republished from The conversation under a Creative Commons license. Read original article.

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