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Bay Area Birds: The mysteries of migration

By Charlie Frisk

One of my wife’s favorite expressions is, “When it comes to biology, never say never, and never say always.”

I can think of few areas where that is more appropriate than migration.

It seems every bird species, and even individuals within a species, do things a little differently than the rest.

Birds leave their winter habitat in the spring to go to areas more suitable for rearing offspring.

Many factors play a role in the areas they choose for nesting, the most important of which is food supply.

Some people believe birds migrate south because they can’t stand the cold.

In reality, most birds are pretty cold tolerant, and they migrate in the fall because their chosen food supply becomes scarce or nonexistent.

The amount of time birds spend on their nesting range varies greatly by species.

One which sometimes spends little time here is the Baltimore oriole.

Orioles typically arrive sometime in May, and some of the adults leave shortly after their young fledge from the nest.

This could be as soon as early July, though the peak of oriole migration is in August and September.

Human behavior has dramatically changed migration patterns for some species.

Historically, almost all of Wisconsin’s bald eagles migrated south when lakes and rivers freeze because they could no longer catch fish.

Many of Wisconsin’s bald eagles still migrate, but with hydroelectric dams providing open water, many spend the winter in Wisconsin.

There are locations below dams on the Wisconsin, Mississippi and Fox rivers where many eagles congregate in the winter.

A few bald eagles even overwinter in northern Wisconsin.

They survive primarily by feeding on road-killed deer, a winter food source that wasn’t available in the days before roads and vehicles.

Famed conservationist Aldo Leopold said, “One swallow does not make a summer, but one skein of geese cleaving the murk of a March thaw, is the spring.”

In Leopold’s time (early 1900s), Canada geese were absent from Wisconsin in the winter, thus they were a sure sign of spring.

Today, we see geese 12 months a year.

The geese we see today are the giant subspecies of Canada geese, averaging 12 1/2 pounds as adults, with some individuals weighing up to 20 pounds.

Because the giants nested in the Midwest in the days before hunting regulations, they were hunted year-round, and their eggs were collected as a food source.

Eventually, the subspecies were declared officially extinct in the 1950s.

But surprise, a small population was discovered in Rochester, Minnesoat, in 1962, and wildlife managers enthusiastically stocked them throughout the Midwest.

The giant subspecies have very little migratory urge.

If they can find sufficient food, they will spend the winter in Wisconsin.

The other subspecies of Canada geese are truly migratory, and as their name indicates, they nest primarily in Canada and most migrate south for the winter.

How birds learn to migrate varies based on species.

In cranes and geese, the parents teach their offspring the migration route.

This adaptation allowed biologists to develop a population of whooping cranes which migrate between Wisconsin and Florida.

They accomplished this with ultralight airplanes made to look like whooping cranes.

Young cranes hatched in incubators were trained to follow the airplanes.

Today, Wisconsin has a whooping crane population in both Necedah and Horicon National Wildlife Refuge.

There are only about 600 whooping cranes left in the wild and captivity.

In common loons, the adults leave the northern lakes about a month before the juveniles, wintering along the Atlantic or Gulf of Mexico coasts.

The juvenile loons evidently have the migratory route innately programmed into their brains, because they are on their own for the migration.

For the next couple of months, keep your eyes peeled for birds migrating through Wisconsin.

The showiest migrators are the geese, cranes, ducks and shorebirds, but if you watch carefully, you will see warblers, true sparrows and many other smaller birds moving through your backyard.

Migration is one more sign fall is on the way.

No bread, please

Most of the geese found at Bay Beach Wildlife Sanctuary are giant Canada geese.

Feeding the geese has become popular, but a problem occurs when people feed bread to the geese.

Although they like the bread, it does not provide any nutrition.

Geese that eat too much bread end up with a grotesque disorder called angel wing, because the wings stick straight out.

These geese cannot fly and are doomed to a slow death.

So next time, leave the bread at home.

Editor’s note: To read another Bay Area Birds article by Charlie Frisk, CLICK HERE.

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