Journey North Whooping Crane Spring 2006

Meet Tom Stehn

March 22, 2006

Dear Journey North,

It is hard to say how many whooping cranes have started the migration. On our March 22 census flight, I only found 163 whooping cranes at Aransas out of the estimated 215 in the flock. However, skies were dark all day with high clouds covering the sun. Under those difficult viewing conditions, I never can find all the cranes.

Some Gone, But It's Still Early
I estimate between 20 and 30 whooping cranes may have started the migration, no more than 14% of the flock. It is still early for the bulk of the whooping cranes to head north. I do have some evidence that the migration has started:

One whooping crane was present March 11–16 on the Platte River in Nebraska. We believe this was the whooping crane that wintered with sandhills in extreme south Texas until around March 3. Because it was with them, it migrated more on the earlier schedule of the sandhill cranes.
A birdwatcher reported seeing 3 whooping cranes northwest of Fort Worth, Texas on March 19. This is right on the expected migration path of the cranes, and his observation sounded credible.
Combined with the low number of cranes found on the census flight and knowledge that whooping cranes can leave in late March, it seems likely that some are currently flying north.

Why Don't Whoopers Migrate Together in Flocks?
Why don’t the whooping cranes all migrate together in large flocks? Wouldn’t that help them watch for predators and find the route to Canada? Perhaps, but there are advantages to the cranes migrating in small groups and leaving at different times. Birds that leave too early may find less food in the colder conditions of early spring and encounter bad snow storms such as the blizzard that dropped 2 feet of snow on Nebraska on March 20. Cranes that leave too late might get to Canada late and miss the best time for nesting, perhaps when there is less food available for their newly hatched chicks. The process is kind of like the expression “you learn from your mistakes." The cranes that migrate at the right time get rewarded with higher survival and better reproduction. Thus, the population—through the process of natural selection— “learns” when to migrate. You might say they “know” when to migrate, but it is really the process of natural selection that determines when the best time to migrate is.

WANTED: Fresh Water and Blue Crabs
Conditions are tough for the whooping cranes at Aransas right now, with few blue crabs to eat and high water salinity, forcing them to fly inland to get fresh water to drink. On the flight, we found 24 of the whooping cranes inland at freshwater ponds and dugouts.

Two days ago, I walked to numerous fresh water ponds looking for crane tracks and droppings. From the material I picked up, I found mostly shells about the size of the fingernail on my smallest finger. Think how many of these tiny shelled critters the whooping cranes would have to eat to get enough calories. Think about all the grinding up of the shells their stomachs would have to do to digest all that material to get the small bits of meat! As I said, conditions are tough for the cranes right now.

Tom Stehn
Whooping Crane Coordinator
USFWS