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'Field Notes:' How Scientists Study Bird Migration

Bar-tailed godwit.

Migration is one of the many adaptations used by birds and other animals to cope with the cold temperatures and scarcity of food that winter can bring. As scientists and naturalists we are interested not only in where birds go in the winter, but in how we know where birds go in search of more hospitable conditions. Traditionally, scientists captured, tagged and released individual birds and hoped that someone, somewhere, would find this bird and report its whereabouts.

In recent years, however, satellite transmitters have given scientists an unprecedented opportunity to track birds around the globe. Once a bird is equipped with a transmitter, it emits signals that are picked up by space satellites and reported to a computer back on earth. Using this technique, scientists have documented a peregrine falcon migrating from Alberta, Canada, to Mazatlán, Mexico. They have tracked swallow-tailed kites from Florida and Georgia to previously unknown wintering grounds in Brazil over 5,000 miles away. A Swainson’s hawk traveled from California to the pampas of Argentina, where it joined a gathering of thousands of other Swainson’s hawks, including hawks that had been banded in California, Colorado, and Saskatchewan, Canada.

Some birds engage in truly epic migratory flights. Biologist Robert Gill has studied bird migration for over 30 years. In 1976 he was studying migratory birds on the southeastern coast of Alaska. He was impressed by the obvious fat stores on a small wading bird, the bar-tailed godwit. Scientists knew that bar-tailed godwits spent their winters in places like New Zealand and Australia and probably took a series of flights through Asia to reach their destinations. Gill wondered why this little shore bird had such a heavy layer of fat and wondered if this allowed the bird to stay in the air for longer flights than other birds.

For 30 years Gill managed as best he could, building a network of bird-watchers who looked for migrating godwits over the Pacific Ocean. Finally, in 2006, technology caught up with Gill’s hypothesis. He and colleagues implanted satellite transmitters in bar-tailed godwits and tracked their flight. Nine satellite-tagged bar-tailed godwits departed Alaska in the fall. The transmitters sent their location to Gill’s computer. He sometimes stayed up until 2 in the morning to watch the signals on the Google Earth program running on his laptop. The bar-tailed godwits flew south across the Pacific Ocean. They did not stop along the way. Instead, they traveled up to 7,260 miles in nine days — the longest nonstop flight ever recorded. Godwits leave Alaska between late August and early November and generally wait for a large storm system with strong southerly winds to start their journey. Gill clocked average speeds of 35 miles an hour for the migrating godwits, with storms, gusts and cyclones boosting them south.

These long journeys can be incredibly stressful. Some birds do not feed during their migratory journeys and must rely on stores of fat--and antioxidant-rich berries--to see them through to journey’s end. Despite the incredible amounts of energy expended by birds in their epic migrations, the primary advantage of migration is actually conservation of energy.

Birds generally fly farther north (or farther south, south of the equator) to take advantage of the explosion of vegetation that accompanies long summer days farther from the equator. The extended daylight hours allow diurnal birds (active by day, sleeping at night) to produce larger clutches than those of related non-migratory species that remain in the tropics year round. As the days shorten in autumn, the birds return to warmer regions where the available food supply varies little with the season.

As the days lengthen and the return of spring is ever more imminent, the vocalizations of these winged travelers will again greet our ears. If only we could understand those voices—for they just might tell a tale or two of their migratory adventures great and small.

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Field Notes," 3/16/14, 3/17/14 and 3/20/18.  Listen on-air or online Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or via podcast.)

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