Sandhill Cranes Subject of Ongoing Research Investigations

By James Ed. Ducey.

Flocks of the gray Sandhill Cranes and a myriad of waterfowl visiting the central Platte River this season continued to be the focus of bird research.

The Platte River Whooping Crane Trust conducted annual surveys of the wild birds. Ground counts about assigned routes documented the cranes and waterfowl weekly from early March to mid-April. Flights in a fixed-wing airplane were also done from Chapman to Overton, several times during this period.

Although cranes were present at the end of January, the first wave of cranes arrived around the second week of February” said Dr. Daniel Kim, avian ecologist at the Trust. “The real influx did not occur until early March when the cranes arrived en masse.”

Kim has experienced crane migration for four years, describing the shear number of cranes as overwhelming, noting that “when in a blind with 40,000 cranes just outside, their calls are at least half the experience!”

Peak numbers on the river for Spring 2007 were estimated at 300,000, not including cranes already off roost, which, depending on the morning, may be as many as 40% of all birds.

“From a wildlife perspective, this is one of the greatest migratory events on earth,” Kim said. “The resources, corn and the river, draw 500,000+ cranes that winter from New Mexico to Louisiana. Some birds add an addition 1000 miles to their migratory route just to use the Platte.”

“In general cranes settle the eastern part of the valley first, with roosts filling in to the west. Cranes leave roosts in the same order, that is they leave the eastern part of the big bend first. The cranes display a combination of hard wiring and migratory flexibility that influences when they migrate.”

The amount of water in the river influences the roosts used by the cranes. “Years with an abundance of water appear to distribute cranes throughout the river while low water levels seem to restrict cranes to a few favored roosts,” Kim said.

River management appears to have profound effects on crane use of riverine roosts. Due to low flows, the river must now be maintained, or it would be choked with vegetation such as cottonwood saplings and willow thickets, and the channel would be obstructed with perennial vegetation bank to bank, reducing the river's ability to absorb flood waters. Both channel clearing and tree removal appear to encourage roosting cranes.

Large roosts are associated with channel maintenance, while unmanaged reaches of the river receive very little crane use.

In general, cranes off the river use corn fields to a greater degree than all other land types (foraging), but the largest groups use wet meadows for loafing and social interactions.

“Poor weather during the end of March and beginning of April held many cranes through April 12. Two very fine days led to a mass exodus by the 16th.”

During their visit, local weather has a huge influence on when migratory peak occurs, typically between the second and fourth week of March, Kim said. Some years cranes trickle in and trickle out.

Aerial and road surveys conducted by the Trust are considered an index of crane abundance. Surveys have been conducted since 1998. The aerial surveys have not occurred during high flows, so the impacts of a full river have not been documented, Kim added.

Survey results indicate that the Central Flyway population is steadily growing at about 3-6% annually, according to surveys documenting crane population increases by the U.S. Fish and Wildlife Service, Kim said. Crane trust surveys are not used to calculate population numbers, but provide important information needed to help manage cranes.

During their seasonal staging, Sandhill Cranes interact very little with the large numbers of geese present, according to the research.

“There is very little direct interaction,” Kim said. Both species may co-occur on the same field, but very few antagonistic behaviors are observed. Geese prefer to roost on ponds and lakes, while cranes appear to prefer running water. Also, when there is water in the western rain water basins, geese congregate in the basins, when the basins are dry, geese spill over into the Platte valley.”

Investigations into other aspects of Sandhill Crane biology are underway along the central Platte. Current research topics are:

1) Questions of competition for food (corn) between cranes and geese. While not direct, geese generally arrive 2 weeks prior to cranes, Kim said. “Some scientists believe geese may deplete waste corn, forcing cranes to either forage further from the river, or leave the valley in poorer condition.”

2) Landscape level questions are being addressed by a Ph.D. student at UNL, but meaningful conclusions are still at least two years off.

The U.S. Fish and Wildlife Service undertakes seasonal counts of the Whooping Crane along the central Platte valley. Dr. Kim said the number of documented whooping cranes seen in the Platte valley was a highlight of the past season.

There are concerns for migratory birds in the central Platte valley.

Kim mentioned the interaction between crane watchers and the birds: “(1). People try to get too close to cranes, whether on the road, or by visiting the river. If people want an up close experience with the cranes, they should use one the guided tours available through either Crane Meadows or Rowe Sanctuary. (2) People here to visit cranes are visitors, and simple etiquette needs to be observed … Respect private lands, and if you pull off the road to see cranes, make sure that both farm equipment as well as cars can pass.”

As regional climates may change, Kim said “the impact for cranes may be due to increased variations in both temperature and precipitation. The lack of predictable conditions, especially precipitation, may constrict a large number of cranes into ever decreasing spaces in the river. The greatest potential impact to cranes will Sandhill Cranes likely track favorable conditions (water availability) during the winter. Prolonged drought in the southwest and northern Mexico could have severe impacts to winter crane populations.”

River management is conducted by a partnership including U.S. Fish and Wildlife Service, Nebraska Game and Parks Commission, The Nature Conservancy, National Audubon Society, The Platte River Whooping Crane Trust, Prairie Plains Resource Institute, and dozens of private land owners, Kim said. Most of the funding is secured through grants, some private, others federal. The Nebraska Environmental Trust has been a major source of funds for river management.