Crane Migration Update: March 13, 2001
Are They Leaving Already?
Dear Journey North,
On my last weekly aerial census flight March 8 on which I tried to find all 176 wintering whooping cranes, I came up WAY SHORT. I only counted 154! I did find all 7 family groups (two adults with their chick hatched last summer), and found about 40 other territorial pairs.
I felt fairly confident that one territorial pair was definitely missing and may have started the migration. In all, an estimated 10 whooping cranes may have departed the wintering area and headed north.
Whooping cranes normally don't start the migration until the last week in March or first two weeks in April. It would be unusual if 10 whooping cranes have started north this early. However, food continues to be short here at Aransas. The blue crabs just aren't available for the cranes to eat, so they are forced to eat clams. Imagine swallowing clams whole and relying on your gizzard to grind up all that shell and salvage a little bit of nutrition from the little piece of meat inside a clam!
Are you ready for a Challenge Question?
See our graphing activities and data analysis tips using the whooping crane population data from 1940 to the present. After the winter's low crane count, it is a good reminder of the tenuous status of this endangered species.
Why are there fewer cranes this year? Last week's report got us all thinking about this question. (See Discussion of Challenge Question #1, below.) With today's report, the plot thickens. Brian Johns, Wildlife Biologist, said low rainfall last summer at Wood Buffalo National Park in Canada led to parts of the nesting area being very dry by mid August. Brian had this to say about last year's low numbers in the breeding season:
"The total known nesting attempts in 2000 was a record 51. By mid June many of the wetlands in the Klewi River area were dry, making the young cranes more vulnerable to predation as the crane families searched for aquatic foods. With 51 nesting pairs it was anticipated that 40-50 young would hatch. However, only 30 young cranes were accounted for. Surveys began about 10 days after the first chicks hatched so there were likely additional chicks hatched that were not seen. Two pairs hatched two young (twins). Losses of young continued over the summer so that by mid August only about 13 young were still alive. The families with twins had each lost one of their chicks by mid August as well. The number of nesting pairs that successfully bring a young to Texas has varied from a low of 12% to a high of 75%, with the average of about 46%.
What Do You See? Challenge Question #6
Teacher Tip: Two Timely Lessons
Food supply has been a big part of the cranes' story this year. Perhaps your students will now stop to think about how the food chain changes with the seasons. This lesson will help your students see the central role that sunlight plays in living systems.
Why Fewer Whooping Cranes? Discussion of Challenge Question #1
"Why are there FEWER whooping cranes in the population this year? What different reasons can you find that explain why the number is so low?"
Tom Stehn's full report described the effect of the drought Texas had last spring and summer. The drought caused a shortage of blue crabs, the cranes' most nutritious and favorite food. A shortage of fresh water flowing into the bogs changed the salinity, cutting down blue crab reproduction. Not having enough of the highest quality food in their diets may have affected the cranes' winter health, as well as their abilities to successfully nest. To make the natural drought even worse, human uses took much of the water from the rivers, reducing the water that reached the bogs. It's a situation that threatens to get worse as more people move to Texas and place more demands on the water supplies.
You're the Scientist: Answers to Challenge Question #2
"Which TWO of these calls are given by the SAME pair of cranes?"
This question took a good ear, and we're cheering for Kenneth and also Chris, Michela, Brian, and Melissa from grade 7 at Iselin Middle School. They all said calls #1 and #3 are given by the same pair of cranes.
That's exactly right! Dr. Wessling will be proud of you!
Just in Time: Discussion of Challenge Question #3
Voiceprints are a valuable discovery for safe, non-invasive monitoring of cranes. Thanks to the work of Dr. Bernhard Wessling, we now know that individual voices can tell us which crane is which. Voiceprints are reliable ways to identify cranes without expensive banding, stressful capturing, or risk of losing ID bands. This photo shows Dr. Bernhard Wessling with a wind-proof microphone. The shaggy mane over the microphone eliminates wind buffeting and provides for excellent quality recordings of the crane calls.
How to Respond to Today's Challenge Questions:
1. Address an e-mail message to: email@example.com
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