South for the winter?

arctic tern - <em>photo by <a target="_blank" href="http://www.flickr.com/photos/ianworsley/">Ian Keith</a></em>It’s January in Montana. It’s cold outside, there’s snow on the ground, and most of our birds have gone south for the winter.

Or have they? We say birds fly south “for the winter,” but in many cases, when they get to where they’re going, it will be summer there too. January in Argentina is summer for the Swainson’s hawks that share our Montana summer in July. They get two summers each year. With the help of modern technology we can track where such long-distance migrants go and understand a little more about their lives.

Scientists can attach a GPS backpack transmitter that sends the bird’s position up to a satellite. To keep the weight down there’s a trade-off between the number of data points that can be transmitted, and how many months before the batteries run down. The smallest GPS units have solar panels (the size of your thumb nail) to recharge the batteries, but even these units weigh 100 grams or so, and you can’t put that on a small bird.

But now there are really tiny geolocators that weigh a mere 1.6 grams and can be attached to a bird’s leg. Geolocators record the time of sunrise and sunset each day, and the interval between the two can be used to calculate the bird’s location. But they don’t transmit the information, they store it internally (for up to 20 years!) so the researcher has to go find the bird, when it returns to its nest site, and remove the geolocator in order to get the data out and do the calculations.

Arctic terns breed in the Arctic and travel to the Antarctic and back each year. This is the longest known migration – the round-trip distance is roughly 44,000 miles. An international team of researchers attached geolocators to 60 arctic terns and analyzed their migrations, from Greenland to the far South Atlantic and back. After leaving the breeding grounds the terns congregate for about 25 days in a small patch of the N. Atlantic (opposite Newfoundland, or approximately 80° due east of Missoula!), then head south again to the Cape Verde Islands off the coast of Africa. From there most of them continue along the African coast, but a few cross the Atlantic again and fly down the coast of South America. They all converge in the Weddell Sea for the “winter” (read: summer). The southbound migration takes 3 months, and the journey back north is 40 days. Details of the migration routes and the lightweight geolocators can be found on the Arctic Term Migration Project‘s web site.

Sooty shearwaters also migrate huge distances, about 40,000 miles round trip, but in the opposite direction. They breed on the coast of New Zealand (October thru March) and travel north “for the winter”, as it were, to the coasts of Japan, California, and Alaska – and spend another summer there. Scientists at UC Santa Cruz attached 6-gram geolocators to 33 birds and retrieved 19 of them. They discovered that the shearwaters tour the whole expanse of the Pacific Ocean in a huge figure 8, and they are on migration for about 200 days each year. The longest distance traveled in one day is about 550 miles. For more information, there’s an interesting article about this in the UCSC magazine.

The Falcon Research Group, from near Seattle, put GPS transmitters on a dozen peregrine falcons in Chile and tracked their migration to the high arctic. These units transmit positions three times a day, potentially for years. One bird—they named her “Island Girl”—just recently completed her third migration cycle, nesting on Baffin Island in NE Canada, at 63°N, and returning each year to the same beach where she was tagged, at 34°S. On her northbound journey this year she covered 9,500 miles in 55 days, the longest day’s flight being about 300 miles. The FRG web site has interactive maps for each of the peregrines they tagged – you can see their position every day, and where they roost each night.

These three species migrate intermittently, stopping along the way. By contrast, some species fly huge distances non-stop. The grand prize goes to bar-tailed godwits that nest in Alaska. Researchers from USGS and Massey University fitted GPS transmitters to 16 bartailed godwits in New Zealand, hoping to find out details of their journeys. The batteries were expected to last a couple of months at the most, but one unit transmitted long enough to cover the migration in both directions. This particular bird flew from NZ (19°S) to China in a single flight of 6,340 miles, and after a few weeks there she continued on to her breeding grounds in Alaska. In late August she left Alaska (59°N) and flew non-stop back to New Zealand, covering 7,100 miles in a little over 8 days. She returned to the same mud flat she had left from 5 months earlier. For more details see this article on the USGS web site.

Birds are amazing!

by Pat Little


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