At the Aldo Leopold Foundation, we call them our neighbors, as each fall a large portion of Central Wisconsin’s sandhill cranes congregate on the islands and sandbars in the Wisconsin River near the Leopold Shack. For the past seven years, I have been leading tours that allow visitors to view these annual crane congregations. One of the challenges posed to tour participants has been to “stump Stan” with a question about cranes that I can’t answer. I’d been doing well until last year, when someone asked if there was a special term for a large group of cranes. I drew a blank, but a quick search online revealed that there is a term that traces back hundreds of years: a sedge of cranes.
What brings this large sedge of cranes to the vicinity of Leopold’s Shack? After their breeding season, sandhill cranes follow a predictable pattern as they prepare to migrate. My student, the late Scott Melvin, and I described this pattern after studying sandhill crane migration 35 years ago. In late summer, breeding pairs and their offspring start losing their territorial defensiveness. They begin to spend time foraging with other nearby family groups and non-breeders before returning to their home territory each evening. Eventually, usually by late October, most sandhill cranes leave their summer habitat and converge on migration staging areas, which are typically within a single day’s flight from home.
These traditional staging areas, as the term implies, are used year after year by large numbers of cranes as they ready themselves for the upcoming, longer distance migration. Parent cranes lead their offspring to these places, and the young then return to them in subsequent years, maintaining the tradition through many generations. These regional staging areas always offer the co-occurrence of two essential requirements: an abundant source of food (usually waste grain available after the harvest) and nearby nocturnal roosting sites (often on an island) where the flocks of cranes feel safe from nighttime threats.
Our stretch of the Wisconsin River, sitting within the Leopold-Pine Island Important Bird Area (IBA), fits the bill perfectly. River islands and sandbars provide ideal nocturnal roosts, and nearby cornfields provide abundant waste grain. Furthermore, the surrounding region of central Wisconsin is the core area for breeding cranes in the state. The crane staging area was indeed one of the key justifications for the IBA’s creation in 2005.
The daily routine for the assembled cranes consists of leaving the roost in the morning and heading out to nearby foraging areas before returning to the roost at sunset. An evening roost can contain thousands of cranes packed into a particularly desirable area. Once they have congregated at a staging area, they will remain there throughout the fall until the food supply is no longer available and winter weather sets in, usually around mid-December. Typically, the departure coincides with either deep snow burying waste grain in the fields or freezing temperatures icing over the river.
That often more or less synchronous departure marks the start of the cranes’ steady migration southward to a wintering area. Cranes use a very efficient method of flying while migrating. They find a rising thermal of air and spiral upward to altitudes of several thousand feet without expending much energy by flapping their wings. As the lift provided by the thermal dissipates, the birds peel off on a long descending glide in the direction they want to travel, often with a tail wind. Again, that long glide path involves expending little energy flapping their wings. The process repeats itself, allowing cranes to cover hundreds of miles very efficiently.
Along the way, the birds will pause at migration stopovers. These are not visited traditionally like staging areas and are often selected opportunistically at the end of a day’s flight. The stays at stopovers are usually short. Eventually, the fall migration is completed when cranes reach areas where they traditionally spend the winter. In the past, those traditional wintering areas were as far south as Florida, but as winter climates have gotten milder further north, cranes are stopping short to spend the winter.
If you’d like to witness our impressive sedge of cranes this fall, please consider joining me for a crane congregations tour. And as in the past, attendees will be challenged to try and stump me with a question about crane biology!
Feature photo, top, courtesy of Sunil Gopalan.