Migration’s mysteries

“Nature’s Compass: The Mystery of Animal Navigation,” James L. Gould and Carol Grant Gould, Princeton University Press, 2012, $29.95

The more we learn about how animals migrate, the more complicated and awe-inspiring it seems.

A red-eyed vireo leaves Canada in the fall and turns up in a tree — possibly the same tree it left last spring — in the Amazon basin five weeks later. A monarch butterfly sets out in September on a trip it has never taken before, and ultimately joins millions of its kind in a small forest on a Mexican mountain. The natural world is full of journeys like this, feats that humans, for the most part, cannot accomplish without specialized equipment and instruments.

In “Nature’s Compass,” we learn that many of a migrant’s strategies may be innate but are far from simple. For one thing, many migrating animals cover thousands of miles, requiring complex internal maps. As they travel they need to continually amend their sense of where they are in the world, often using sensory information not available to humans, such as UV light, subsonic sound, even far-off scents.

They factor in time elements, plane geometry, spherical geometry, internal compassing and several other kinds of navigation, to go from point A to point B. In the case of birds, youngsters that have never before made their species’ migratory journey have a strong innate sense of how to travel, but can alter their behavior in response to contingencies, such as unfavorable winds or abnormal temperatures.

Humans increasingly are disrupting migratory paths for many species by altering the physical environment. Birds and others use their capacity for change to adjust as well as they can: Many short-distance migratory birds now arrive significantly earlier in the spring at our latitude, as the growing season advances due to climate change.

Better understanding of how birds migrate is helping in efforts to restore some endangered species, such as the whooping crane and the cahow. As this book clearly shows, we don’t have all the answers, but the better we understand how songbirds, whales, sea turtles, butterflies and others make their amazing journeys, the less we will impede them.

Val Cunningham