Migration worth the trip for tropical birds

Baltimore orioles are tropical birds. So are most of the colorful species that take up temporary residence to nest here. They are tropical species that make long and dangerous migrations because the risk is worth the reward.

In the tropics, yearlong uniform weather offers a constant level of food for birds — constant but not abundant. Birds nesting in the tropics typically produce fewer chicks per nest than do, say, that oriole and his mate nesting here. So tropical birds work harder to raise fewer young.

There is enough food and territorial space in the tropics, however, to allow winter migrants to subsist until spring.

Why return? Why invest the energy and test the dangers of migration twice a year? Why not settle for a smaller clutch of eggs?

The onset of warmth in northern North America creates an explosion of plant and insect life offering superior opportunity to raise young. A bird can double the number of chicks per nest, and perhaps nest more than once. More young equals a more profound genetic heritage.

Migration begins when a signal sets a hormonal clock inside the bird. A timed sequence begins. The bird might molt, fatten for migration, migrate, court, nest, raise young, molt again, fly south — some or all of the above.

The clock stops at a point, then resets.

Author Scott Weidensaul wrote about migration in his book "Living on the Wind." The birds, he says, "have been drawn from their tropical havens by the subtle shifts of the photoperiod and the biological clocks within their own bodies."

Humidity or rain or something else also could have a role. Perhaps there is a combination of signals. Perhaps it is just that internal clock, ticking through the year, one hormonal change leading to another.

Migration is a complex behavior that has developed over millions of years. It is thought to have existed as far back as Gondwanaland, the earth's original single landmass that broke into the continents we see today.

In North America, glaciers came and went, the environment changing with the advance and retreat of ice. Cold weather eliminates food and water sources for certain species of birds, like orioles, tanagers and warblers.

Birds moved south when the continent was cold, returned to residence in the north when it warmed again. Migration became etched in their genes.

If needed, birds can override the migration program on either a temporary or permanent basis: Temporary, for instance, when a storm blocks a migration route; permanent should conditions change year-round needs.

That flexibility hopefully will play well for them as our now restless climate undergoes its own evolution.