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Bay Area Birds: Mysteries of Migration

By Charlie Frisk

The experience of fall migration is very different from that of spring migration.

In the spring there is the excitement of all the first sightings – the first redwings, the first white pelicans, the first sandhill cranes – until all of our favorites have returned.

In the fall we end up wondering, when did I last see that species?

My wife Kathy asked me about a week ago when I last saw a white pelican.

Sandhill cranes. Sue Ascher Photo

We both thought it had been about two or three weeks, but we weren’t really sure.

That is the way it is with most fall migrants – they leave sometime in the night, with no fanfare.
A few species do give us a little warning.

Barn swallows perch on the power lines in flocks of hundreds before heading south.

Sandhill cranes abandon their strict territoriality and group together in large flocks for a few weeks before they migrate.

The complexity of migration and the skill with which it is accomplished is one of the many marvels that make birds so interesting to study.

There are so many mysteries to migration.

Biologists have a laundry list of the cues that birds and insects use to navigate their migrations, but there is no real consensus as to exactly what navigation tools individual species depend on.

There have been many studies performed in which birds have been moved to locations hundreds of miles off their normal migration route, yet they still end up in the correct location.

This would indicate that birds have both an internal compass and an internal map.

It is thought that most birds use celestial cues, either the sun or stars, and some species likely use both.

It also appears that many birds utilize the earth’s magnetic field.

The magnetic field is probably the most important cue for migrating insects such as butterflies and dragonflies.

Some bird species are thought to use the patterns of polarized light.

These species typically start their flights at sunset or a little after and use the polarized light patterns to provide information for their initial migratory flight pattern.

Birds, bats and insects show up on radar when they are migrating.

Monarch butterfly. Nancy Nabak Photos

Many species of migrating birds take off in the evening, fly through the night and land by sunrise.

On particularly clear nights with no moon, birds that utilize the stars can navigate most effectively, and radar images will be filled with migrating flocks.

In some bird species, adults and juveniles migrate together.

In cranes and geese, the adults teach the juveniles the migration route.

In 2001, when conservationists decided to re-establish whooping cranes in Wisconsin, they determined that the best winter range for the whoopers would be in Florida.

They used ultra-light airplanes that the young cranes had been conditioned to follow to lead the cranes from Wisconsin to Florida.

In other species, the adults will leave before the juveniles.

With those species, the migration route has to be innately programmed in the young birds.

Common loons are an example of this, the adults typically leave about a month before the juveniles.

I had noticed that when the ruby-throated hummingbird numbers began to drop in our backyard that I wasn’t seeing any adult males.

I researched this, and it turns out the adult males leave first, followed by the adult females and finally the immatures.

It is thought that by leaving earlier, the adult males won’t be competing with the females and immatures for fading fall blossoms and other food sources.

The immature hummingbirds need extra time to mature and gain fat reserves before flying off on their first migration.

Every elementary school child knows that monarch butterflies migrate, but not many people know that several species of Wisconsin dragonflies also migrate.

A couple of weeks ago, a brother of mine who lives in western Iowa called to tell me that for the past few days, he had hundreds of dragonflies in his yard.

He wanted to know if any species of dragonflies migrated.

I looked it up and found that Wisconsin’s most common migrating dragonfly is a large species called the green darner.

Other Wisconsin species that migrate are the black saddlebag, red saddlebag and variegated meadowhawk.

From the description he gave, it appeared that most of the dragonflies passing through his lawn were green darners.

Just like the monarchs, most of Wisconsin’s migrating dragonflies end up in Mexico, but some travel farther on to Central America.

Both monarch butterflies and dragonflies migrate in the daytime in large enough swarms to show up on radar.

Wisconsin never gets its original monarchs or dragonflies back.

The monarchs migrating south in the fall remain sexually immature throughout the migration and overwintering.

Upon starting north in the spring, they become sexually mature and will soon mate, lay eggs and die.
This can happen several times on their way back north.

Wisconsin’s migrating dragonflies will mate, lay eggs and die on their winter range, so it is a different generation that begins the trip back up north.

They, too, will undergo multiple generations during their migration.

The different species of duck are some of the showiest migrants.

Two excellent locations to see a variety of migrating ducks are Bay Beach Wildlife Sanctuary on the south shore of Green Bay and Barkhausen Waterfowl Preserve on the west shore.

We certainly have a lot better understanding of migration than even the best scientists had in the past.

The Greek scientist Aristotle thought that some birds changed species for the seasons, and he suggested that common redstarts turned into European robins in the fall.

For centuries people believed that birds such as swallows and martins that lived around water burrowed into the mud at the bottom of lakes and slept through the winter.

In the 1700s scientist Charles Morton proposed that departing birds flew to the moon in the winter, and he even calculated that it would take about 60 days for them to fly there.

I think it is kind of exciting that although we now know much more about migration, there are still many mysteries yet to be solved.

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