More on the miracle of migration; a rare loon visits

Written by Jack Sanders
Thursday, 19 November 2009 17:06

Several people have told me stories about how they have looked out a window in May and seen a hummingbird flying around the very spot where, the year before, a nectar feeder had been hanging. Nothing else drew the bird to that spot — except the memory of a food source it had used many months and thousands of miles before. Clearly, those birds were returning to precisely the same location they had left seven or eight months earlier.

While the ability to fly may seem the most amazing thing about birds, migration is much more astounding. With a brain that weighs a tiny fraction of an ounce (the whole bird weighs only a tenth of an ounce), a hummingbird can negotiate 2,000 miles of land and ocean and find a small spot of the world that was its home the summer before. I wonder if any human could accomplish a similar feat — with no maps, no roads, no signs, no GPS, and no gas stations at which to stop and ask directions.

To navigate between summer and winter homes,  scientists believe that birds use many techniques including the sky, earth’s magnetic fields, landmarks, scents, sounds, as well as an extraordinary memory.

Experiments have shown that some birds use the sun in daytime to determine direction while those that migrate at night use the stars. These experiments usually involve birds that are caged indoors. In the case of nighttime migrants, scientists can employ a planetarium with stars projected onto the ceiling. By modifying the night sky — removing various stars — Cornell professor Stephen Emlen found evidence that species such as the Indigo Bunting may use constellations to guide their way. Some birds use both sun and stars, taking note of the position of the setting sun for general direction and then using stars as guides.

To prove that some birds use the earth’s magnetic fields like a compass, scientists have strapped small magnets onto pigeons. The fields produced by the magnets confuse the pigeons’ detection system, and they cannot find their way. In another experiment, birds were placed in a cage that had its own magnetic field. By reversing the field, scientists were able to get birds to reverse their direction.

Experimenters have also attached special goggles that blurred the distant vision on homing pigeons, which could use other techniques such as magnetic fields to get close to a destination, but then had trouble finding local landmarks once they got near their destination.

In 1952, Rosario Mazzeo described a Manx Shearwater, which was transported from Great Britain to Boston by airplane and then set free. Twelve days later, the bird had returned to its nesting site in Wales, more than 3,500 miles away. Some years later, L. Richard Mewaldt, an ornithology professor at San Jose State University in California, described an experiment in which several hundred sparrows were taken from San Jose to Louisiana and set free. At least 15 returned to San Jose where they were recaptured. The 15 were then transported to Maryland and at least six of them made it back to San Jose!

To migrate long distances, birds need not only a sense of direction but also a sense of destination. How does a bird remember exactly where it lived many months earlier? Even more remarkable, how does a bird born in its breeding grounds find its way to a winter residence it has never seen before?

In The Proceedings of the National Academy of Science, Claudia Mettke-Hofmann  and Eberhard Gwinner of the Max Planck Research Centre for Ornithology reported in 2003 that migratory birds have evolved better memories than birds that remain in one place year round. In the process, they have also evolved bigger hippocampus regions of the brain. The scientists put two different but similar species of warblers — one migratory and the other non-migratory — into a pair of vegetation-filled rooms. One room had food and the other did not. The two scientists found that the migratory warbler was able to remember where the food was for up to 12 months while the non-migrating bird forgot within a couple of weeks.

Next week, why many prefer to migrate at night.

Coming Up

In Search of Birds of Prey,  hawks, owls, eagles and more, photographed by Jim Zipp of Hamden over 35 years, Thursday, Nov. 19,  7 p.m., free, Western Connecticut Bird Club, at Kensington Green, 655 Main Street South, Southbury, 203-426-3901.

Mother Nature’s Bird Foods Hike, wild foods that birds like, Saturday, Nov. 21, 1 to 2:30, Audubon Greenwich, 613 Riversville Road, RSVP 203-869-5272 at x221.

Owl Dinner Detectives, kids can dissect sterilized owl pellets, Saturday, Nov. 21, 3 p.m., $10, Discovery Center, at Ridgefield Recreation Center, 195 Danbury Road (Route 35), 203-438-1063.

Copyright (c) 2009 by Jack Sanders. Send sightings or comments to: jackfsanders [at sign] yahoo.com, or to Bird Notes, Box 1019, Ridgefield, CT 06877; or call 203-438-1183, extension BIRD (2473), and leave a message with your report, spelling your first and last names and telling us your town. If you need help identifying a bird, try your local nature center. If you find an injured bird, call wildlife rehabilitator Darlene Wimbrow of Redding, 203-438-0618, Wildlife in Crisis of Weston, 203-544-9913, or Wild Wings of Greenwich, 203-637-9822. The columnist’s website is www. sandersbooks. com.