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Platte River Ecology Study


Synopsis


Each spring about 80% of the sandhill cranes in North America stop for several weeks in the Platte and North Platte River Valleys of Nebraska while en route to breeding grounds in Canada, Alaska, and the Soviet Union. This concentration of cranes is unparalleled in North America and these sites represent the principal spring staging areas for the midcontinent population. Large numbers of waterfowl, numerous bald eagles, whooping cranes, and many other species also use the area.

Recent changes in habitat conditions along the Platte and North Platte Rivers have prompted concern for the welfare of sandhill cranes and other migratory birds found there. With approximately 70% of the Platte's annual flows diverted for various consumptive uses upstream in Colorado, Wyoming, and western Nebraska, channel width in many areas has been reduced to 10-20% of former size. Habitat conditions within the existing channel have also changed as a result of reduced scouring of sandbars and shifting of alluvial sediments. A broad band of mature deciduous woodland now occupies tens of thousands of acres that formerly were part of the river and numerous islands overgrown with woody vegetation exist within the channel. Concurrent with diminishing surface flows in the channel, the water table beneath adjacent native grasslands has declined, thereby facilitating extensive conversion of these sites to cropland.

In 1978, the U.S. Fish and Wildlife Service began a 3-year investigation to define habitat-use patterns and habitat requirements of migratory bird populations utilizing the North Platte and Platte River Valleys in Nebraska and to assess factors influencing woody vegetation establishment along these Rivers. Concurrent investigations were conducted by the U.S. Geological Survey and the U.S. Water and Power Resources Service to identify flows necessary to maintain desired levels of migratory bird habitat and locate potential sources of water to be utilized in habitat maintenance.

Research findings indicate that sandhill cranes occupy four discrete staging areas encompassing approximately 1037 km in the Platte and North Platte River Valleys. Individual staging sites (identified by nearby towns and cities) and their size (in km2) are as follows: Kearney-Grand Island, 578; Overton-Elm Creek, 257; Sutherland-North Platte, 184 and Lewellen, 18. Cranes typically begin to arrive in late February and some remain into late April; peak numbers are present from about 20 March to 5 April when up to approximately 500,000 cranes are present. About two-thirds of the staging population is situated along the Platte River between Kearney and Grand Island. Cranes generally remain within 1.6-3.2 km of the channel during the staging period and rarely move beyond 6.4 km from the River. Individuals average five moves per day with daily flight distance averaging about 9.7 km. During the course of the stopover period, individuals roost over about an 11.8-km reach of river; the average shift from one night to the next is about 1.6 km. The cumulative size of an individual crane's home range averages approximately 36 km2.

Sandhill cranes utilize several habitat types in meeting their nutritional requirements. Radio-marked cranes spent approximately 55% of the daylight hours in cropland, 27% in native grassland, and 15% in hayland. In cropland, cranes feed almost exclusively on waste corn, but invertebrates (earthworms, snails, grasshoppers) constitute most of their diet in native grasslands. Both invertebrates and alfalfa leaves and stems are consumed in haylands. About 96% of the total daily intake of foods (dry weight basis) comes from cornfields and 4% from native grasslands and hayland. Cranes do not compete with livestock for corn because cattle feed on ears whereas cranes feed primarily on scattered kernels. The birds forage for invertebrates to secure certain essential nutrients either lacking or present in low levels in corn. Ingestion of animal matter balances the diet by supplying certain amino acids and minerals. Calcium needs are met from snail shells and from concretions unearthed in the calcareous loess soils on native grasslands.

The Platte River Valley is a major fat deposition site for the midcontinent sandhill crane population. It is estimated that over 90% of the fat accumulated during the staging period is derived from corn. Males and females deposit fat at daily rates of 13.2g and 9.1g, respectively, during the stopover period. The amount of fat stored varies directly with the length of time individuals stay at the Platte. Approximately 40% of the fat reserves are utilized during the long migration; most of the remaining reserves sustain the birds during the nesting season.

The food requirements of the sandhill crane population are substantial; estimated daily food intake ranged from about 0.3 metric tons of corn on 1 March to 57.3 metric tons on 31 March. The cumulative corn intake for projected populations of 350,000 and 450,000 cranes was estimated to be 1023 and 1315 metric tons. Current harvesting techniques leave approximately 6-7% of the corn crop in the fields; cattle consume approximately half of that during the fall and winter months. At current population levels, cranes utilize 10-20% of the corn available when they arrive. Corn availability is adequate to meet the population's current energy requirements; however, the near total dependence on corn for this purpose causes the population to be vulnerable to food shortages if land use changes.

The "Big Bend" reach of the Platte River historically received relatively high use by whooping cranes during migration. This reach continues to be used, but at markedly lower levels than in former periods. Habitat degradation associated with channel shrinkage is probably a major cause of this decline. Whooping cranes roost in broad reaches of river channel with most documented use occurring at sites between 155 and 365 m wide. Foraging occurs in a variety of habitats but agricultural lands receive the highest use. The whooping crane, like the sandhill crane, ingests both plant and animal matter during migration.

Thousands of mallards and Canada geese spend each winter in the Platte River Valley. The birds gather at ice-free sites along the River, particularly between Lexington and Grand Island. In late winter, large populations of white-fronted geese, pintails, and certain other species of waterfowl also gather along the Platte River; these early migrants and the overwintering birds move into the rainwater basins as ponds open in spring. Habitat use by mallards during winter varied with the severity of weather conditions. Mallards spent most of their time in the river channel during favorable weather but shifted to nearby canals during adverse conditions. Mortality was substantial during periods of heavy snow and ice cover as ducks fell prey to raptors confronted with diminishing alternative prey. Mallards obtained most of their winter diet from cornfields corn accounted for 97% and 94% of the diets of drakes and hens, respectively. The birds were dependent on livestock to uncover food supplies when snow cover was heavy; prolonged feeding flights and reduced physical condition resulted from diminished foraging opportunities. Although mallard nutritional requirements during winter were met largely from plant foods taken in cropland, small amounts of animal matter were ingested during prolonged feeding periods in riverine wetland habitat. The diet of migrant mallards during the spring staging period changed only slightly from winter.

Most of the midcontinent population of white-fronted geese spend several weeks in south central Nebraska during spring. The population utilizes the area to prepare physiologically for migration and reproduction. The birds rely primarily upon the abundant supply of waste corn to meet maintenance energy requirements and for fat storage. Corn accounted for 77% and 91% of the diet of adult males and females; green shoots of winter wheat formed the other principal component of the diet. Male and female geese acquired fat at a daily rate of 12.5 and 17.6 g, respectively, while staging on the study area.

The presence of extensive tracts of woodland in the Platte and North Platte River Valleys during recent times probably has benefited several species of raptors. Sixteen species of raptors were observed during the study, including the endangered bald eagle and peregrine falcon. The bald eagle was the most numerous raptor; peak wintering populations were estimated to be 150 and 250 birds during the 1978-79 and 1979-80 winters. The other principal raptors, in declining order of abundance, were the red-tailed hawk, American kestrel, marsh hawk, rough-legged hawk, prairie falcon, ferruginous hawk, and golden eagle. Densities of raptors along established survey routes were 0.46 birds/km in the winter of 1978-79 and 0.39 during 1979-80.

Bald eagles are a common winter resident of the Platte and North Platte River Valleys. Discharges from Kingsley Dam and return flows from the Tri-County Diversion Canal keep certain reaches of the river ice-free during winter, a condition which attracts a sizable overwintering mallard population and allows eagles access to fish. Fish and mallards were major prey of bald eagles wintering along the Platte. Bald eagles occurred throughout the entire 327 km of river under study, but densities varied widely, probably in response to varying food availability; nine roosts were located between Kingsley Dam and Chapman. The diet of eagles was diverse; 23 species of birds and 22 species of mammals were identified from 2858 regurgitated pellets. Eagle food habits were strongly influenced by prey availability; mallards became a major prey during periods when fish were not available because of ice cover or water depth. Most mallards occurring in the eagle diet apparently were pirated from other raptors.

The breeding avifauna of the Platte and North Platte River Valleys and adjacent lands is diverse. A total of 142 species of birds, representing approximately 70% of the species that nest in Nebraska, were recorded breeding on the study area during 1979 and 1980. Thirty-four species accounted for approximately 95% of the total population. Species which nest in riparian woodland habitat have increased markedly because of the presence of extensive woodlands in the former channel of the Platte and North Platte Rivers. The least tern and piping plover nest on open sandbars at sites protected by a water barrier; diminished flows have caused elimination of much of their nesting habitat and the numbers of pairs of both species have declined accordingly. The replacement of about 70% of the native grassland of the Valley with cropland has brought on a corresponding reduction in the abundance of species which breed in grassland, including the upland sandpiper, bobolink, and grasshopper sparrow. Species diversity and density of breeding pairs were high on river channel islands and in lowland forests, moderate on native grassland, and low on alfalfa hayland and cropland.

Analyses of annual tree rings and other data indicate woody vegetation encroachment on the channel occurred principally after Kingsley Dam on the North Platte River and the Tri-County Diversion Canal on the Platte River became operational during the early 1940's. West of Overton, since 1950, woody vegetation development in riparian woodlands has been confined primarily to the establishment of understory red cedar, willow, and Russian olive. Between Overton and Chapman, the only reach where cranes still stage along the Platte River, substantial encroachment by cottonwood and willow has occurred during the past 30 years. Cottonwood and willow invasion of channel habitat represents the most immediate threat to continued use of the river channel by cranes. The extent of woody vegetation establishment in the channel each year varies with magnitude of flows during the summer months when seed dispersal is in progress. Cottonwoods and willows usually release their seeds during June and July. After release, cottonwood and willow seeds remain viable for about 20 and 10 days, respectively. Seedling establishment by these species generally occurs on fine textured soils with a high percentage of moisture and a long substrate exposure period. After becoming established, if the seedlings grow to a height of 15 cm, which can readily occur during the first growing season, the plants can withstand prolonged inundation the following spring. Therefore, maintenance of adequate flows during the summer months to prevent seed germination and seedling establishment represents the most effective means of controlling woody vegetation encroachment in the channel.

Sandhill cranes roost in the shallows and on sandbars in the river channel. These birds prefer roosting in areas where the channel is at least 150 m wide, and strongly avoid river channels narrower than 50 m. The height of vegetation along river banks and on islands influences the selection of roosting areas when unobstructed channel width falls below 150 m; the presence of a bridge or a road adjacent to the channel also reduces use. Of 406 0.8 km segments of river channel within the study area, 137 (34%) were utilized as roost sites in 1979. Crane densities at 18% of the sites exceeded 5000 individuals/0.8 km. The loss of about two-thirds of the original roosting habitat between Kingsley Dam and Grand Island has caused crowding at remaining roost sites, thereby increasing crane susceptibility to catastrophic losses from natural forces, particularly severe storms and disease. Four diseases with epizootic potential have been reported in migratory birds either on the study area or within populations that occur there; these are botulism, duck plague, avian cholera, and aspergillosis. Heavy losses of several species of waterfowl to avian cholera occurred in the Rainwater Basin Area a few miles from crane staging areas during the period of study; in 1980, 30,677 dead waterfowl were retrieved from the disease die-off area.

Native grasslands, river channel, and cropland each serve a vital role in maintaining sandhill crane use of the Platte and North Platte River Valleys. Both native grassland and channel habitats are threatened by declining flows. The rapid rate of attrition of native meadows, where cranes obtain much of their protein and calcium, poses an immediate concern. Prompt action is needed to protect key grassland tracts near the channel. Crane requirements are well suited to a habitat maintenance plan that protects tracts of moderate to small size spaced a few miles apart along those reaches of the two Rivers that cranes still inhabit, instead of creating a large refuge near Grand Island as originally proposed. Additional protection, possibly through easements, will be needed to maintain existing land use practices on other native grassland tracts near the channel between the managed reserves. Maintenance of an unobstructed river channel width of 150 m at sites within existing staging areas is adequate to satisfy roost-site requirements of the sandhill crane population during the staging period.


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