Charlottesville, Virginia
March 30, 2021
I recently listened to a podcast on this topic. Neither the podcast nor this article address why these birds migrate so far. The podcast addressed that these birds are incredibly efficient in their oxygen use, using over 90 percent of the oxygen they take in which allows them to fly at heights of up to 40,000 feet. By comparison humans use less than 10 percent of the oxygen we take in.
Here’s the article about these massive migrations:
Flight is one of the most astonishing of avian attributes. Speeding falcons, graceful egrets, and the aerial acrobatics of courting hummingbirds leave me in awe.
Perhaps the most astounding aspect of flight is one that we cannot see — birds’ ability to cover distances, sometimes incredible distances. Consider the Blackpoll Warbler that breeds in the boreal forests of Canada and winters in the Amazon basin of South America. That’s a hefty jaunt.
But it’s the truly long-distance migrants that challenge believability. The Arctic Tern is the world’s champion long-distance migrant. It breeds in the circumpolar Arctic and sub-Arctic and winters in the Antarctic. Tracking studies have found the birds make annual journeys of about 44,100 miles. One tern flew from the Netherlands to Antarctica via New Zealand, for a one-year trip of 57,000 miles. Arctic Terns don’t make the trip without stopping, of course, but this fact doesn’t detract from the total distance traveled.
Arctic Terns are long-lived; many survive 20 years, and one banded bird lived for 34. To put this into perspective, consider that thousands of Arctic Terns have an annual migration almost twice as far as flying around the earth at the equator. A tern that lives for 22 years, for example, would fly more miles (970,200) than the equivalent of two roundtrips to the moon in its lifetime (955,600 miles).
Another distance champion is the abundant Sooty Shearwater. It breeds in New Zealand and spends its “winter” at productive feeding sites throughout the northern Pacific Ocean during our summer. It is virtually impossible to determine how far any one bird flies during a calendar year. Impossible, that is, without high-tech equipment.
Research biologist Scott Shaffer from the University of California at Santa Cruz led an international team that trapped 19 Sooty Shearwaters in New Zealand and attached miniature geolocation tags that measured the birds’ position, temperature, and dive depth.
After the breeding season, the birds flew to Antarctica, north to Chile, across the equator, and then north primarily to one of three nutrient-rich areas near Japan, Russia, or California. During their “spring” (our fall), the shearwaters returned to New Zealand to breed.
The shearwaters spent about 200 days away from New Zealand. Their migratory path was roughly the shape of a giant figure 8 and measured about 40,000 miles.
From New Zealand to Alaska
A project undertaken in 2006 and 2007 to learn more about the potential role of migrant shorebirds in spreading avian influenza was initiated by a team headed by Nils Warnock, who is now at Audubon Canyon Ranch in the Bay Area of California, and Robert Gill, United States Geological Survey in Anchorage, Alaska. Advanced monitoring techniques resulted in unexpected and incredible migration data.
Electronic satellite transmitters were surgically implanted into 14 female Bar-tailed Godwits in New Zealand. (Solar-activated devices were attached to the smaller males.) Females and their transmitters were of greatest interest.
One female stayed in New Zealand, and six others either turned around or their transmitters stopped working, but the seven remaining godwits flew 6,300 nonstop miles to the Chinese and Korean coasts of the Yellow Sea in about seven days and stayed for about two months. Five then flew approximately 4,000 more miles nonstop to the Yukon-Kuskokwim Delta in Alaska. The sixth flew to eastern Russia, and the seventh ended up in Alaska after first visiting Russia.
To everyone’s surprise, some of the transmitters worked longer than expected. On August 29, 2007, a female (E7), left Alaska on its fall flight to New Zealand, with her transmitter blazing. After passing close to Hawaii, she made a slight turn and continued south, flying directly to New Zealand, landing near the mouth of the Piako River on the evening of September 7. The landing was slightly more than eight days from leaving Alaska, and the site just eight miles from where she had been captured and fitted with the transmitter seven months earlier. Unbelievably, her flight of about 7,200 miles was also nonstop, and, at the time, the longest nonstop ever recorded.
Godwits do not soar. E7 flapped constantly for the entire 7,200 miles and likely did not feed or drink, and probably did not sleep. It is estimated that she utilized energy at a rate eight times her resting rate and maintained it for eight days.
Nine years after the godwit study, in 2016, Anders Hedenström of Lund University in Switzerland reported in the journal Current Biology on a tracking study of 13 Common Swifts that breed in southern Sweden and winter either in western or central Africa.
The swifts spent more than 99 percent of their 10-month non-breeding season airborne, and three of the birds never landed at all. Those that did stop over the 10 months only did so for an hour or two at a time. Stunning.
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Birds that remain in constant flight still need to sleep. Hedenström doesn’t know for sure, but he speculates that the swifts sleep on the wing, possibly during daily dawn and dusk ascents to altitudes of 10,000 or more feet.
Researchers have learned that digestive tracts of long-distance migrants work overtime prior to migration. Food intake is increased, and the food is metabolized and stored as fat, which serves as the birds’ flight fuel. Bar-tailed Godwits store fat until it makes up about one half their weight. Then the digestive system shuts down so metabolic energy can be maximized for muscle use.
The ability of birds to master marathon flight allows them to migrate long distances, if necessary, to accommodate optimum habitats for both breeding and over-wintering. This has led to astonishingly long flights and reminds us again of the amazing behavior of birds.