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Conditioning of Sandhill Cranes During Fall Migration

Discussion


G. c. rowani and G. c. canadensis subpopulations staging during fall in central North Dakota undergo major gains in body mass and fat content from early September to late October, after which most depart from the state. Sandhill cranes probably exhibit similar patterns of body mass and fat gain on other fall staging areas in the northern plains region. At Last Mountain Lake, Saskatchewan, the fat content (expressed as a percentage of dry tissue mass) in sandhill crane carcasses increased 3-fold from mid-August to late September 1981 (Tacha et al. 1985). Cereal grains are the principal foods taken in fall in North Dakota (Madsen 1967) and Saskatchewan (Tacha et al. 1985), and are the primary source of nutrients for fat synthesis.

A comparison of rate of fat gain among G. c. rowani staging in North Dakota during fall and in Nebraska during spring suggests fat is acquired more rapidly in spring. The estimated daily rate of fat gain by adult G. c. rowani in North Dakota (10.1 1.4 g, n = 20) was 38% less (P = 0.088, t-test) than the 16.3 2.6 g (n = 34) in Nebraska (G. L. Krapu, U.S. Fish Wildl. Serv., unpubl. data). Causes for the slower daily rate of fat gain in fall are unknown but greater disturbance during the fall hunting season might be a contributing factor. The change in average body mass prior to 1977 versus 1977 and after coincided with a shift in hunting seasons from November to September. Earlier hunting resulted in greater hunter activity and probably reduced the time available for cranes to forage and rest. Although the daily rate of fat gain was lower among G. c. rowani in North Dakota, fat levels at departure were similar in North Dakota and Nebraska because sandhill cranes stayed longer in North Dakota.

Annual variation in body mass of sandhill cranes in North Dakota suggests that the rate of fattening is sensitive to local environmental conditions. Widely dispersed, suitable staging habitat must be maintained so cranes can adjust their fall distribution in response to hunting pressure, food, and roost-site availability. Staging during fall in North Dakota presently is restricted primarily to areas having large shallow alkali lakes with soft bottoms (Soine 1982) where cranes roost at night. Daylight hours are spent mostly on surrounding agricultural lands (Melvin and Temple 1983). Formerly, cranes staged extensively in the Missouri River Valley of North and South Dakota, but most of their roosting habitat was inundated with the filling of several mainstem reservoirs (Buller and Boeker 1965). Alkali lakes are relatively secure from conversion to cropland in the northern Great Plains because high salt concentrations make these sites poorly suited for agricultural production (Stewart and Kantrud 1971). However, alkali lakes can be altered in other ways that reduce or eliminate their value as crane roosting habitat. For example, some alkali lakes have been drained for construction of irrigation canals (U.S. Dep. Inter. 1979b), and proposals have been advanced to freshen and deepen others, currently supporting major crane roosts, to enhance their value for recreation (U.S. Dep. Inter. 1979b). Pumping groundwater for irrigation from aquifers that discharge into alkali lakes can lower the water table and thereby diminish or eliminate the surface waters (Winter 1988), causing loss of crane roosting habitat. Sandhill cranes also are displaced by human disturbance in the vicinity of their roosts (Krapu et al. 1984). Because most of the remaining sandhill crane roosts in the northern Great Plains are vulnerable to various forms of human activity, we propose that sandhill crane use of fall staging areas be regularly monitored and reported to help reduce further habitat loss.


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