Threats to Wildlife and the Platte River

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WILDLIFE p. 13 In the spring, the cranes usually inhabit the Platte valley from mid- February to early May with most individuals staying an average of one month (cranes stop only briefly in the fall, and in lower numbers) (USFWS 1981, pp 19 -20; Iverson et al. 1987). During their stay in the Platte valley, Sandhill Cranes roost at night in river shallows and forage during daylight in adjacent wet meadows and agricultural fields. Their diet during this time consists mainly of waste grain. More than 90 percent of their food (by weight) is obtained from fallen corn, which provides readily digestible energy. Removal of this waste is welcomed by farmers, although the cranes remove only 10 -20 percent of the available corn ( Krapu et al. 1982). Historically, wet meadows were probably the primary foraging habitat of Sandhill Cranes in the Platte region, but over the years much of this habitat has been lost to groundwater drawdowns and cropland conversion. While the cranes are now heavily dependent upon agriculture for survival, they spend a third of their day in the remnant wet meadows, feeding on invertebrates, other animals, and calcium -rich nodules which they find in the soil to obtain essential proteins and minerals which cannot be provided in sufficient quantities from corn (USFWS 1981, pp 24 -25; Krapu et al. 1982, p 547). Cranes often lose more energy than they gain when foraging in sites other than cornfields, a fact which underscores the need cranes have for nutrients they must obtain from wet meadows (USFWS 1981, p 28). Food is often in short supply upon arrival at their breeding grounds in the far North, and survival and successful egg formation depend on the utilization of stored fat, proteins, and calcium. While in the Platte valley, the cranes store an estimated 34 -44 percent of the bodily reserves they will use during migration and reproduction ( Krapu et al. 1985). The fat reserves of adult females upon arrival at the breeding grounds (an average of 480 grams /individual) are largely utilized during the nesting period, which indicates the importance of food availability and habitat quality in pre- serving the species. Accumulation of substantial reserves on the staging grounds prior to breeding gives the female energy to search for foods containing high - quality protein needed for egg formation (USFWS 1981, p 29). Crane distribution during staging is strongly influenced by the width of the river channel. Increased plant growth and channel narrowing in the Platte and North Platte rivers has caused the pattern of river use by cranes to change in recent times. Habitat deterioration already has caused Sandhill Cranes to abandon major reaches of the North Platte and Platte rivers. Currently, cranes are restricted to only a few stretches of river, leaving the remainder of the river uninhabited by the species (USFWS 1981, p 16). At sites where they do gather, they do so in dense aggregations, at times exceeding 10,000 individuals per half -mile of river. About two - thirds of the spring staging