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Omnivorous, exploiting subsurface food items by probing with bills; glean seeds and other foods on the surface (Walkinshaw 1949, 1973; Tacha 1987). Feed primarily on land or in shallow marshes with emergent vegetation.
Mid-continent Sandhill Cranes spend 1.4% to 6.4% of their diurnal time searching for food, 0% to 17.1% gleaning surface foods, and 17.9% to 60.4% probing for subsurface foods (Tacha et al. 1987a). Frequency of these foraging activities does not vary among age, sex, or social classes, but juveniles spend 25% more time foraging than adults, females 2% more time foraging than males, and adult pairs and family units 14% more time foraging than lone adults (Tacha 1987).
Foods vary widely depending on what is available in various seasons and locations (Walkinshaw 1949, 1973). Cultivated grains are major food items whenever available (Appendix 1). Diets during migration and wintering roughly similar among age, sex, or social classes.
Diverse foods consumed at more northern latitudes and during breeding season (Walkinshaw 1949, 1973). In Alaska, breeding cranes eat assorted berries and small mammals during the breeding season (Walkinshaw 1949). In n. Michigan, primarily berries and insects during summer. Nonmigratory subspecies eat insects and their larvae, snails, reptiles, amphibians, nestling birds, small mammals, seeds of various plants, and berries.
On the Copper River Delta of Alaska, cranes arriving on breeding areas eat mostly tubers (Cyperacesae) and gastropods (Iverson et al. 1982). Diets of adults and young in se. Wisconsin include invertebrates and some small mammals and reptiles during early brood rearing; nonbreeding adults eat mostly tubers of aquatic plants in spring and early summer, cultivated grains in late summer (Walkinshaw 1973). Mullins and Bizeau (1978) found Sandhill Cranes eating primarily timothy corms and insects during summer in Idaho.
Nutrition And Energetics
Animal matter important in diet even though it rarely exceeds 5% to 10% of intake by volume or weight; may provide essential amino acids and calcium rarely available in grain (Reineke and Krapu 1986). Carcass protein content of adult mid-continent cranes varies little throughout the year (Iverson 1981, Krapu et al. 1985). Lipid reserves vary dramatically; lowest lipid levels recorded mid-Aug when cranes arrived at Last Mountain Lake in Saskatchewan (Tacha et al. 1985a), but birds gained an average 9 g lipid/d while eating high-energy grains. Lipid levels did not vary from when cranes departed Saskatchewan, through staging in Oklahoma and wintering in w. Texas, to spring arrival in Nebraska (Iverson 1981, Tacha et al. 1985a, 1987a). These cranes deposited an average 12.8 g lipid/d while in Nebraska during spring migration, maintained these lipid levels through spring migration in Saskatchewan and central Alaska to nesting areas in w. Alaska, and then lost 4 g/d during prenesting (Iverson 1981, Krapu et al. 1985, Tacha et al. 1987a).
Accumulation of lipids during fall in Saskatchewan and spring in Nebraska possible because high-energy food resources (grain) are concentrated and accessible (Tacha et al. 1987a). Lipids maintained during later stages of spring migration by birds continuing to stop and exploit grain fields and increasing foraging time compared to Nebraska. Losses of lipids during prenesting due to increased energy expenditures associated with low food availability, territorial defense, and egg production. Krapu et al. (1985) hypothesized that availability of cultivated grain allows mid-continent cranes to transport maximum lipid levels to nesting areas; once there, lipids are used for early reproduction, allowing increased productivity compared to pre-agricultural development in the Great Plains and Alaska.