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Tom Stehn's Report: Questions and Answers
April 4, 2008
Meet Tom Stehn
Whooping Crane Coordinator, U.S. Fish and Wildlife Service

As you read Tom's report this week . . .

  1. How does Tom explain why cranes migrate in small groups?
  2. What are the disadvantages of migrating in small groups?
  3. Do cranes ever migrate in larger groups?
  4. When do the majority of the whooping cranes depart Aransas on their journey north?

Dear Journey North,

The first five of the 266 Whooping Cranes at Aransas (say Uh RAN Zus) left on migration March 25, and several others soon soon followed. The Lobstick adults with their twin chicks have apparently started migration sometime in the past week. They are almost always some of the earlier migrants. So somewhere, a group of 2+2 is en route. The unbanded family group that winters close to them has also started the migration. I plan to fly over the refuge to count the remaining cranes on April 10.

Until then, you may recall that last week I asked you to think of reasons why it would be an advantage for whooping cranes to migrate in small groups. Did you think of these advantages and disadvantages?

Advantages of Migrating in Small Groups
Migration that occurs in small groups is a way of spreading out the risks of all the hazards species encounter when they are in migration. If there are multiple small groups of cranes migrating separately, perhaps only one or two groups will encounter difficulties. If all the cranes migrated together and encountered trouble, such as a bad spring blizzard, the entire population could be wiped out.

Disadvantages of Migrating in Small Groups
The disadvantage of small groups is that the cranes cannot rely on others . . .

  • to stay on course during the migration, or
  • to watch for predators when they stop for the night during the migration.

Each crane must be able to survive on its own. Fortunately, cranes can do this, and some young cranes will actually complete the migration on their own. They sometimes get help by joining up with sandhill cranes in flight or at a migration stopover site.

Sometimes Cranes Surprise Us
Most of the time whooping cranes migrate in family groups or bachelor groups. This general rule helps us determine credible sightings reported by citizens. If they report a large group of cranes flying together, we usually believe they are sandhill cranes. But it seems there's always an exception to every "rule" as this sighting report proves:

Click to enlarge this sighting report.
Photo Willard Gudgel

Heads Up
The vast majority of the whooping cranes leave Aransas the first 2 weeks in April, with peak departures normally between April 4-12. Keep your eyes on the skies!


Tom Stehn
Aransas National Wildlife Refuge

 

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