Every spring, tourists from around the world flock to Nebraska to watch over half a million migrating sandhill cranes (Grus canadensis) roosting on the Platte River. Where the birds choose to rest on this landscape depends on channel width, bank vegetation height, access to cornfields and proximity to human disturbance, a recent study found.
“Cranes prefer wide channels,” said Gary Krapu, co-author on the paper published in the April issue of the Journal of Wildlife Management. “As the channel narrows, the height of the vegetation becomes increasingly important.”
Spanning five migrations from 2003 to 2007, Krapu and his fellow researchers examined over 6,300 roost sites in south-central Nebraska for more than 300 cranes bearing VHF transmitters. At these locations, the biologists measured the features of the channel, river bank vegetation height, distance to cornfields and presence of disturbances such as roads, bridges and houses. They examined the cranes’ roost-site selection by comparing used sites with available ones to figure out the most effective ways to enhance the habitat for the birds.
“When crane managers look at where they should put effort, channel width and bank vegetation height are two factors they can control most through clearing woody vegetation,” Krapu said.
The study indicated that cranes chose wider areas of the river with shorter bank vegetation to maintain an unobstructed view that minimizes the chances of predators ambushing them in the channel, he said. The narrower the channel, the more the birds showed a preference for sites with shorter bank vegetation.
“Cranes will select narrower channels when the amount of corn ground near roosts increases,” Krapu said.
Cranes get almost all their energy from corn and use it to store fat to refuel for migration, he said, and in preparation for breeding across the vast, frigid territory from western Quebec to 1,000 miles into arctic Russia.
Krapu and his team noted that the wary birds avoided regions impacted by roads and other human disturbances in the vicinity.
“At distances over 300 or 400 meters, if you can improve the channel by manipulating channel width and bank vegetation height, cranes will accept disturbance to a greater degree,” he said.
These findings supplement insight gained over two decades of crane research in the Platte River Valley on the key environmental factors influencing roost site selection in sandhill cranes, Krapu said. The results can help crane managers decide where to allocate their resources as they work to maintain adequate roosting habitat for cranes in this critical spring staging area.
“This should be a guide to where managers will get the biggest bang for their buck,” he said.
|Julia John is a science writer at The Wildlife Society. Contact her at email@example.com with any questions or comments about her article.|