Tradition of the Sandhill Cranes
Joseph T. Springer
Professor
Biology Department
University of Nebraska at Kearney
It is estimated that between four hundred thousand and five hundred thousand sandhill cranes (Grus canadensis) stage along the Platte River each spring and have been doing so for thousands of years. “Staging” means that the birds are congregating, either prior to or in the middle of migration. During this time, the sandhill cranes will consume enough extra food to finish the migration, to successfully lay and incubate two eggs, and to rear at least one young.
Such a concentration of any cranes species only occurs along the Big Bend region of the Platte River in central Nebraska. Certainly other crane species stage prior to or during their spring migrations, but nearly all cranes species number in the hundreds or low thousands, limiting the size of staging flocks to less than one hundred. The closest thing to what is seen in Nebraska is the staging of common cranes every spring in Spain, but their numbers are only in the hundreds.
Feeding
We know that today corn comprises ninety percent of the sandhill cranes’ diet. Since field corn is the result careful hybridization, self-planted corn is not generally viable. Although it may grow into a mature plant, it will either produce few or no seeds. Either way, it is of no practical use to the farmer and is considered a weed. Thus, most farmers are just as happy to see their fields full of cranes in the spring.
Knowing the importance of corn for sandhill cranes, several students from the University of Nebraska at Kearney examined the impact of crane foraging on waste corn. They sampled fields in early February to find how much corn had been left after the fall harvest. Then they sampled the same fields two or three times after the cranes arrived to see how much of the available corn the cranes consumed. The students questioned the farmers to determine if they had seen flocks of geese, ducks, or crows in the fields. We discarded data from fields where other birds had been seen feeding. The results in Table 1 reflect how thoroughly cranes remove waste corn from a field. Nearly ninety-five percent of the surface corn that had been available in February was gone by early April.
Click on table for full-size view
This brings up a unique characteristic of these sandhill cranes. The regular, total depletion of a food source (in excess of ninety percent of what had been available) is unheard of in the Animal Kingdom. In virtually all natural settings, if a food source were depleted to this extent, any species depending on that food would die out. In fact, before total depletion can occur, the consuming animals would move elsewhere or die off; it would simply be too difficult to forage successfully in the area. The difference here is that the food is artificially replaced by man every year. There is no evolutionary price to pay for eating every seed of their primary food plant.
What of their food habits before corn? Did cranes eat such a high percentage of the seeds of their traditional food plants (probably prairie grasses and flowers)? If so, the plants and/or the cranes would not have survived. The answer probably lies in the fact that cranes do not seem to search as thoroughly for underground seeds. Numbers of corn kernels covered by an inch of soil declined by only forty-five percent during our sampling (Table 1). Seeds of native prairie plants have evolved mechanisms that aid in getting themselves underground. Those that buried themselves would have been fairly safe from the cranes. Those that remained exposed would not have germinated anyway, so their loss to cranes was of no consequence to the plants.
Cranes are extremely traditional. That is, they stay with their parents for a full year, watching their parents and mimicking them. Therefore, even though cranes do not seem to feed on prairie plants nowadays, that does not mean they never did. When the prairies and meadows along the Platte were initially cultivated, cranes that had landed in those areas in the past continued to land there. Even if they had never seen corn before, there would have been none of the seeds that they traditionally ate. It seems reasonable that at least some of the cranes would have tried the new food. Their young of that year would not have known its novelty. It was something that their parents ate, and, therefore, must be food. From then on, it would have been a tradition for that family of cranes and all their descendants.
Migration
Tradition affects migration too. Some sandhill crane populations never migrate: for example, Florida sandhill cranes, the endangered Mississippi sandhill cranes, and the endangered Cuban sandhill cranes. It is not that these cranes cannot fly. They can and do. But their parents never migrated, and the climates are such that there is no overwhelming need to migrate.
Sandhill cranes use four distinct continental flyways. Cranes travel the Pacific Flyway between their wintering grounds in California and nesting sites in Oregon, Wahsington, and British Columbia. In the Western Rocky Mountain Flyway, greater sandhill cranes migrate between wintering grounds at Bosque del Apache National Wildlife Refuge in New Mexico and nesting grounds in Wyoming and Idaho. A third flyway extends from wintering grounds in Florida to nesting grounds in Michigan, Minnesota, and Wisconsin. The greatest number of sandhill cranes use the Central Flyway with the long stopover on the Platte River.
Cranes in the Central Flyway generally spend the winter in Texas and Mexico. After four to six weeks of fattening up in Nebraska, they will move northward, nesting anywhere from Hudson’s Bay in Canada to the eastern parts of Siberia in Russia. That is why the time along the Platte River is so critical; there are thousands of miles yet to fly before nesting can begin.
Each crane learns the route by flying it with its parents the first year. That is why we see so many threesomes flying from one place to another while the cranes visit the Platte. They continue taking the same route for several years with unpaired peers and eventually with their own mates. They will winter within the same ten square miles of Texas or Mexico their parents did, will nest within the same ten square miles Canada, Alaska, or Siberia their parents did, and will stage within the same ten-mile stretch of the Platte River their parents did.
When they migrate southward, they do not congregate as they do in the spring. During migration, cranes fly very high. They fly in relative small flocks, ten to twenty birds, and may land for a few days in any of a great number of areas between the front range of the Rocky Mountains and the eastern edge of the Great Plains.
Cranes are hunted during the fall migration in several states, but the number killed each year is quite small. The fact that cranes are hunted anywhere surprises most Nebraskans. Although the western meadow lark is Nebraska’s official state bird, the sandhill crane is quite likely the bird most Nebraskans consider their own. In terms of wildlife management, there is no reason why sandhill cranes should not be hunted. The death by hunting of a few thousand cranes each year of a population exceeding five hundred thousand is not significant.
Another aspect of tradition among the cranes is that they mate for life. Cranes are the very symbol of fidelity in the Orient, often seen decorating paintings, scrolls, and screens. Fidelity is not limited to cranes, but is common in many species where males and female look alike: geese, swans, and wolves for example. We know that if a mate dies by natural causes, accident, or hunting, the survivor will find a new mate. While researchers have marked and watched thousands of cranes over many years, there has been only one documented case of “divorce and remarriage” in sandhill cranes.
Roosting
The corn is not the primary attractant for sandhill cranes staging in the Big Bend region. After all, there is more corn grown in Illinois and Iowa than in Nebraska. What draws cranes to this region is the Platte River itself because it offers a safe place to roost while diminished fat reserves are replenished. Of all possible threats to sandhill cranes, it is the loss of preferred roosting sites that is the most serious. These sites have been diminishing greatly even over the past ten years, and there does not seem to be any likelihood of changing this trend in the near future. The demand for water from the Platte for irrigation is simply too intense. What once was a wide, shallow river has become a series of narrow, deep channels running through a forest.
Many people think to “roost” means to perch up in a tree. It actually means to rest and/or sleep. Some birds do roost on branches in trees, but sandhill cranes prefer to roost by standing in shallow water that is no deeper than their heels are high. (Crane knees do not bend backward. They stand on their toes with their heels high in the air.) They also prefer that the area for one hundred yards on all sides surrounding them is free of trees, tall shrubs, or anything that might obstruct their view of potential predators.
The first cranes to a roost site in the evening get the best spots. Those that arrive late find only less preferable areas where the water may be a little too deep (still below the belly feathers), too shallow (some will be on land), or too near obstructions. The prime spots themselves will vary daily as the river water levels fluctuate and as the position of sandbars change.
There is further evidence that roost sites have been diminishing. Within the past four years, a few cranes have begun using some of the rainwater basins south of the Platte to roost in. A few have been reported on the South Loup River, about 20 miles north of the Platte, and some on the Republican River, about 40 miles south of the Platte.
ACKNOWLEDGEMENTS
I would like to thank the following students, whose field work on corn consumption added to the content of this paper: Nile E. Kemble, Kevin J. Sabata, Noel A. Timmons, Mark Bernt, Robin Harding, and Larry Klimek.
LITERATURE CITED
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Frith, C. R. 1974. The ecology of the Platte River as related to sandhill cranes and other waterfowl in south central Nebraska. M. S. Ed. Thesis, Kearney State College, Nebraska. 110 pp.
Iverson, G. C., T. C. Tacha, and P. A. Vohs. 1982. Food contents of sandhill cranes during winter and spring. Proceedings of the International Crane Workshop 4:95-98.
Lewis, J. C. 1979. Taxonomy, food, and feeding habitat of sandhill cranes, Platte valley, Nebraska. Proceedings of the International Crane Workshop 2:21-27.
Littlefield, C. D. 1981. Mate-swapping of sandhill cranes. Journal of Field Ornithology 52:244-245.
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Walkinshaw, L. H. 1973. Cranes of the World. New York: Winchester Press. 370 pp.
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