Update from the Whooping Cranes' Winter Headquarters
We've had great migration conditions every day the first 10 days in April. Let me try to describe the conditions that help the cranes migrate and also answer some of the questions I asked last week.
Two things are most important for the migrating cranes. The first is favorable winds. Winds blow from high pressure towards low-pressure areas, so winds change as low pressure, or storm systems, move across North America. The cranes want a wind either behind them or at an angle to their route so the stream of air can push them along. Leaving the coast of Texas, all the cranes so far in April that have migrated have had very strong southeast winds. "Southeast" winds mean they are coming from the southeast and thus will push the cranes to the northwest. Since the cranes' direct migration route is a compass bearing of 332 degrees (north-northwest), the southeast "tailwinds" provide them a strong push at their tail. Several times during their trip north, the cranes will encounter winds from the north or northwest, what one would call headwinds if you were trying to fly to Canada. With strong headwinds, the cranes will stop the migration and wait several days for the winds to turn around and blow from a more favorable direction. Thus, although the cranes may make it all the way to Canada in 12-14 days of flying, the total journey may take 20-25 days depending on how many days they stay on the ground waiting for favorable winds.
The second key factor is thermal currents. When watching the cranes just before they start a migration flight, we always notice "heat waves," or a shimmering of air, visible through our spotting scopes about 15 minutes before the cranes depart. In the morning, as the sun heats up the land, the warmed air rises and provides lift for the cranes. This warming usually becomes strong about 9:30 AM, a typical time when cranes will start their migration flights. The cranes are lifted by these thermal currents and will rise as high as 6,000 feet in the air--more than one mile high. They then glide to the north down to about 1,000 feet before catching another thermal. When rising in a thermal, the cranes make very little forward progress. When they are gliding down, we have clocked them travelling at 62 miles per hour. When strong tailwinds are combined with thermals, the cranes can travel about 420 miles in a day. Usually about 5 PM, the thermals are dying out as the sun gets lower, and the cranes may start flapping their wings more and flying in a straight line, but soon they will get tired of that type of effort and look for a place to spend the night. What characteristics of a place where the cranes will spend the night do you think are desired by the cranes in order for them to feel safe?
Last week I asked if cranes would ever fly above a thick cloud layer the way ducks and geese do. The answer is "No." They will not since there are no thermal currents above thick clouds. Whereas ducks and geese continually flap their wings during migration, and thus benefit from the "thinner" air high above the clouds that offers less resistance, cranes rely on thermal currents and soaring flight patterns. Other large birds such as pelicans, hawks, eagles, and vultures also rely on thermal currents and soar in spiral patterns to migrate.
One other question to answer from last week is: Why would some cranes be more anxious to start the migration than others? The answer comes down to physical differences between adult birds that need to get up to Canada and nest, and non-breeding birds (subadults) that will not be breeding this summer. Subadults are cranes usually between ages 1 to 4, too young to mate. Different hormone levels can be measured in the blood in adults and subadults. Adults have much higher hormone levels, making them anxious to get north and breed. The younger subadult birds don't have those high hormone levels and thus will take their time leaving Texas.
But it's not always that simple. Sometimes subadult whooping cranes will migrate with sandhill cranes and thus be heading north in March in front of many adult whooping cranes. Adults can start the migration at vastly different times. For example, one year we had one breeding whooping crane pair start the migration in early March, and the last breeding pair left Aransas on April 20. That's quite a span of time. You don't have to worry about the cranes that leave late. Those late migrants usually make a more rapid trip to Canada so that all the adult pairs will get up to Canada in time to nest.
Next week I should know just how many whooping cranes are left at Aransas. I'm fairly sure most of the adult pairs will have migrated.
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