"Nature Guide Journal"

14 September 2002

Birdwatchers flock to our coastal shores and wetlands in autumn to observe the flocks of migrating birds.

Coos County birders watch as fall and winter bring an increase in many kinds of ducks, gulls, and shorebirds, as well as a decrease in various kinds of small birds, such as warblers, hummingbirds, and swallows.  These patterns may vary greatly even among related species:  winter wrens are year-round residents here, while house wrens breed in our region but spend winters in the south; Anna's hummingbirds reside in Coos County year-round, but Rufous and Allen's hummingbirds fly south for the winter.

While many animals regularly move from place to place—including certain butterflies, bats, sea turtles, fish, and, whales—birds seem to catch most of our interest.  The seasonal migration most familiar to us is wintering in warm tropical or neotropical environments, then moving north to higher latitudes in the summer to breed.  That migration is so familiar we use it in every-day speech when we call humans who escape cold northern winters "snow birds."

How do migrating birds find their way?

The sun's location in the sky is one bird navigation tool.  However, where you're headed can't be determined simply by knowing the location of the sun:  you also need a clock.  (If you couldn't tell the difference between 9am and 4pm, you'd end up traveling in great arcs.)  Necessarily, sun-navigating birds also have internal clocks.

In fact, the sun also spurs migrants' physical preparation:  birds begin to add fat and become more restless as the day length changes.

Sunlight is polarized by the atmosphere, creating designs visible to at least some birds and used as a navigation tool.

Many species of birds migrate only at night, guided by the stars.  Turns out that it's the relative arrangement of stars that birds use to navigate, not individual points.  And, it appears that star-navigating birds must learn the arrangement.

Like Scouts using a compass, many birds use the earth's magnetic fields to get a fix on direction. Such birds may have physical compasses (perhaps due to iron-rich crystals found associated with their nervous systems) or a special visual ability to "see" the field.

In mapping migration routes of some birds, it's clear that geographic features are also sometimes used to navigate, since some species follow shorelines or trace major rivers during their migration.

Different species of birds use different techniques, and some are better at those techniques than others.

And, each technique has drawbacks—for example, clouds can obscure the stars above or landforms below.  Navigation is most reliable for birds using a combination of several methods.

Navigating during migration appears to be instinct reinforced by experience.  The instinct is clear for species in which first year and experienced birds depart at different times.  (In some species, too, males and females travel separately.)  The value of experience shows up in cases where practiced migrants adjust to being moved far off course, whether forced off-course by storms or by researchers.

Migration and migration routes apparently evolved over many generations, spanning major climatic and geologic change.  It seems those changes are the reason certain species may take different routes on the northern and southern journeys, and routes may or may not be direct.

More than simply "flying south for the winter," seasonal bird migration we anticipate is not as straight-forward as it may appear.