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Tom Stehn Reports from Aransas: April 9, 2002
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Tom Stehn and Survey Plane |
Despite incredibly strong winds on Saturday March 30 (see April 2 report with details and wind maps), I found out from my latest count of the whooping cranes that only 16 had started the migration between March 28 and April 4. Why so few? All they would have had to do on Saturday in that wind storm was leap in the air and I think the wind would have pushed them about a mile. After all, whooping cranes weigh at most around 15 pounds. The peak wind gust recorded that day was 54 miles per hour from the south. Any crane that departed Aransas that day could have crossed nearly all of Texas in one day, probably covering 400+ miles and averaging 60 mph. So why didn't the cranes all migrate?
Even though the cranes don't have suitcases to pack, the answer is simply that the whooping cranes weren't quite ready to leave. As the days lengthen, hormones in the cranes are changing. Those hormones are an important part of migration and the upcoming breeding season. ( Maybe the cranes didn't leave because they watch CNN and know that it just snowed 8 inches in Minnesota.) Through thousands of years of trial and error, the cranes have learned not to leave too early because the far north up where they are going near the Arctic circle is still frozen. So despite the excellent migration conditions March 30, only a handful of cranes departed Aransas.
By having the cranes leave at separate times in small groups, the flock is protected from all running into a natural hazard. What are some of the natural disasters that cranes could face? One for sure is spring blizzards. A few years ago, a spring blizzard on the Platte River in Nebraska killed hundreds of sandhill cranes. Some got buried in snow drifts, and some had their beaks frozen shut with ice so that they couldn't breathe. Another natural hazard is lightning. Fortunately, cranes encountering a storm will simply stand out in the open and wait until it passes. This is a lot safer than for cows that will hide under a tree during a storm, with the tree being the highest object around to attract lighting. A third natural hazard is disease. As thousands of birds migrate north, many of them use the same wetlands and diseases can be spread in the water. Fortunately, most of the water is still cold in the early spring and doesn't yet provide good growing conditions for disease. This is more of a problem for migrating birds in the fall.
None of these natural hazards are as dangerous to whooping cranes as the man-made hazard of power lines. Whooping cranes hit power lines every year and are killed. I just learned there are over 1/2 million miles of transmission lines in the United States. These are the biggest lines high in the air. I don't know if anyone has ever calculated the amount of distribution lines in this county which are equally hazardous to whooping cranes. Anybody want to volunteer to calculate that figure? Distribution lines are the small lines usually right next to roads that bring electricity into your house. Cranes simply don't see power lines, big or small (transmission or distribution), and collide with them in flight.
So I'm still waiting for the majority of the whooping cranes to leave their winter home. I really expect to have less than 100 cranes here when I next try to count them on April 10. So far, an estimated 25 cranes have started the migration out of the wintering flock of 174 whooping cranes, leaving 147 still present at Aransas.
Tom Stehn
Whooping Crane Coordinator
U.S. Fish and Wildlife Service
Copyright 2002 Journey North. All Rights Reserved.
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