Magnets and Migrations: Navigating without GPS
It was mid-September before I got around to taking in their feeder. By that time the pair of Rufous Hummingbirds, and the family they raised in our small corner of Alpine Meadows, were probably already sipping nectar from tropical blooms on the high Mexican plateau. When some of them return next April, as they have for many years past, they will have completed a round trip of nearly 6,000 kilometres.
To duplicate this marvel of navigation a human traveller would require at least a clock, a sextant, and a compass. We know that birds orient on the sun, moon, and stars and, in conjunction with their biological clocks, use these visual clues for navigation. But we also know that many long-distance migrants find their way when the night sky is covered, and when crossing open ocean as the Ruby-throated Hummingbird does in its 800 km non-stop flight across the Gulf of Mexico. It seems that birds do indeed have a built in compass that is independent of visual clues.
Experiments indicate that many migrating species, ranging from birds to turtles, use the earth's magnetic field for navigation. Homing pigeons displaced in closed aluminum boxes have no problem retracing their outbound route back home, even when their vision is restricted by frosted contact lenses. Yet pigeons displaced in iron boxes, which mask the magnetic field, are disoriented and unable to return.
No one knows who invented the magnetic compass but we have a pretty good idea what the prototype looked like: a chunk of magnetite suspended on a string or floated on a chip of wood. By the 11th century both of these early models were being used by mariners who were at last able to stay on course when the sky was covered. Once known as "lodestone," meaning "way stone," magnetite is a common and widespread mineral (Fe 3 O 4 ) which has a strong natural magnetic polarity – a well-defined N and S pole which allows it to serve as a compass needle.
In 1979 James Gould and Charles Walcott found what could be the biological compass – a tiny crystal of magnetite, the same mineral used in early man-made compasses, in the head of a passenger pigeon. Since then new research has shown that biogenic magnetite embedded in nerve fibres is present in a host of other migratory species. In fact the ability to respond to the earth's magnetic field is such a common phenomenon that Robin Baker in his book The Mystery of Migration speculates that future investigations may concentrate on looking for an animal that cannot detect it.
But the mystery of migration remains. Who knows what will guide my pair of Rufous Hummers back from Mexico – the sun, moon and stars, the sights, sounds and smells along the route, or the complex pattern of magnetic variation. Probably all of these things, stored in their tiny brains like the waypoints in my GPS leading the way back home to Alpine Meadows. I'll have the feeder out and waiting.
Upcoming Events :
Sunday, November 18th — Return of the Salmon Festival (Squamish). The Squamish Estuary Conservation Society presents the second annual Return of the Salmon Festival, a celebration of the varied traditions, delicious foods, music, arts and recreation associated with the return of the salmon. The Festival begins at 11 a.m. (rain or shine) at the Sunwolf Outdoor Centre located on the Squamish Valley Road, north of the Squamish Airport and the municipal landfill, near the Cheekye bridge. Included are a Squamish Nation traditional salmon BBQ with bannock, live entertainment, art and craft demonstrations, games and face-painting for kids, and educational displays. There’s no admission charge and the salmon BBQ will be budget-priced (under $6).
Thursday, November 29th — Whistler Naturalists AGM. Our featured speaker for this year’s AGM is Andrew Bryant, Chief Scientist for the Vancouver Island Marmot Recovery Project. The AGM will be held in the auditorium at Millennium Place and consists of two parts: the business portion of the meeting for members only (starting at 6 p.m.), and Andrew’s public talk (starting at 8 p.m.). Please mark November 29th on your calendar and watch this column for more details.
Written by: Jack Souther