Tom Stehn Reports from Aransas: April 2, 2003
|
|
|
|
|
Tom Stehn and Survey Plane |
We had tremendous weather for migration March 31 and April 2. The whooping cranes took full advantage of the strong tail winds and clear skies. My census flight on April 2 showed that 54 whooping cranes (29% of the flock) started the migration in the past week, and that 68 total (37% of the flock) have departed so far this spring. I also made a short migration, leaving work one hour early yesterday to go windsurfing in the winds, gusting up to 30 miles per hour. I just couldn?t sit at the desk any longer. With those tail winds, cranes would be averaging about 60 miles per hour as they migrate. This we know from 20 years ago, when we had radio transmitters on the cranes and were actually following them in airplanes.
The migration appears to be right on schedule since the majority of cranes usually depart April 4-12. I expect most of the cranes to be headed north by the middle of April. This may be a year where it is advantageous NOT to leave early. It is still very cold up north. It was about 10 degrees F April 1, and still very frozen in Wood Buffalo National Park where the cranes will end up about one month from now.
Although cranes had departed from all the main wintering areas at Aransas spread out over 35 miles of the Texas coast, crane departures were definitely not randomly distributed. For example, 3 neighboring pairs had departed from the south end of Matagorda Island, but the next 18 consecutive pairs on Matagorda were all present. Similarly on Welder Flats, 7 neighboring pairs were present, but the two pairs the farthest to the northeast had migrated. This fits previous observations that a pair of cranes starting the migration may influence their neighbors to also start migration at the same time or soon thereafter.
All winter I have been watching a crane with a drooping left wing that had the same injury reported in Nebraska last fall. For most of the winter, the bird has been in a group with one or two other cranes that we call subadults. (Subadults aren?t yet old enough to breed and establish territories.) On my April 2nd flight, the crane with the droopy wing was all by itself. It will be fine as a single, but did the crane it had been staying with migrate? I?m worried that perhaps the bird is reluctant to migrate because of its wing. However, it appears to fly normally, so we?ll see what happens in the coming weeks. The wing does not droop as far down as it did early in the winter, so it has healed some.
I think I already answered above the question I asked you last week about what weather conditions besides tail winds help the cranes the most. My answer would be clear skies. With the sun shining, the land warms up and creates thermal currents that rise off the ground. These rising air currents provide lift for the cranes and allow them to gain altitude a little over 1 mile high above the ground. The cranes then glide to the north, traveling over 60 miles an hour as they descend to about 1,000 feet before they catch the next thermal current. Thus, the cranes migrate by soaring and gliding using their huge wing spans to ride the lifting air.
Cranes don?t fly in a straight line and flap their wings the way geese do. They would get too tired doing that for very long. Thus, sunny skies and tail winds are the two most important conditions the cranes look for when deciding whether or not to migrate farther north. If conditions aren?t favorable, the cranes will just stay put for however long it takes the weather to improve. The weather can?t improve much here in Texas. It is gorgeous. If you live up north, have faith. Warmer weather is coming soon.
Tom Stehn, Whooping Crane Coordinator
U.S. Fish and Wildlife Service
Aransas NWR
P.O. Box 100
Austwell, TX 77950
Copyright 2003 Journey North. All Rights Reserved.
Please send all questions, comments, and suggestions to our feedback form