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36 The opportunity to test cross - fostering occurred at Grays Lake NWR (GL) in Idaho, where studies on the greater sandhill crane had been in progress since 1969 (Drewien and Bizeau 1974). Between 1969 and 1974, over 700 cranes were captured and color- marked for investigations of nesting biology and seasonal movements (Drewien 1973, Drewien and Bizeau 1974). These studies revealed that sandhill pairs would tolerate considerable manipulation without deserting their nests, individual families showed regular seasonal movements, and young birds adopted the movement patterns of their parents. GL is on the western edge of the known historical range of the whooping crane, and many features made it an excellent site to test cross - fostering: (1) the Grays Lake marsh is large and includes excellent crane breeding habitat; (2) sandhill crane nesting densities are high; (3) nesting success ranged from 78 -92 percent between 1969 and 1974, (4) nesting chronology of the sandhills at Grays Lake is similar to that of the whooping cranes in Canada; and (5) carcass analyses of sandhill cranes indicated minimal residues of organochlorines and heavy metals. Furthermore, color marking of the Grays Lake sandhills had demonstrated the birds made one, often prolonged stop at Monte Vista NWR, in Colorado's San Luis Valley, and wintered in the Rio Grande Valley in central New Mexico (Figure 4). This sandhill population thus enjoyed a maximum amount of protection by using national wildlife refuges for breeding, migration stopover, and wintering (Drewien and Bizeau 1978). Drewien and Bizeau submitted a proposal in 1972 recommending use of the Grays Lake sandhills to test cross- fostering of whooping cranes. Following considerable debate and drafting of an environmental assessment, approval for the experiment was secured in 1974 from the Service and the CWS. Beginning in 1975 and continuing through 1988, whooping crane eggs (216) from WBNP were transferred to GL for placement under marked pairs of sandhill cranes (Table 7). Between 1976 and 1984, eggs (73) from the captive flock at PWRC were also transferred to GL. Details of the cross - fostering experiment have been discussed elsewhere (Drewien and Bizeau 1978, Drewien and Kuyt 1979, Drewien 1975 -1983 and Drewien and Brown 1984 -1990, Unpubl. Prog. Rept. Nos. 1 -25, Whooping Crane Transplant Experiment, Idaho Coop. Wildl. Res. Unit, Univ. of Idaho, Moscow). Many of the initial questions raised by this technique (i.e., will proper migratory traditions be established? Will the cross - fostered whoopers adapt to the obvious habitat and dietary differences, etc.) have now been answered, and field observations indicate that behavioral incompatibilities between the two species generally prevent mixed species pairing and subsequent hybridization. A rather low rate of release was achieved at GL due to small numbers of fertile eggs in some years and excessive mortality of young before fledging. In 1976, 1977, 1978, 1981, and 1986 -1988 drought conditions prevailed during the brood - rearing season. Low water levels and dry conditions reduced available food supplies, and allowed coyotes access to large sections of the marsh. Most chick mortality can be attributed to inclement weather at the time of hatching, poor habitat and food conditions during some years, and coyote predation (Drewien and Bizeau 1978, Drewien et al. 1985). Sandhills at GL suffered similar reductions in productivity during these same years. Although subadult and adult mortality rates have been much lower, a number of birds have been lost to fence and powerline collisions (Brown et al. 1987), disease (Snyder et al. 1987, 1992, Stroud gt al. 1986), predation (Windingstad et al. 1981, Drewien et al. 1989), and other causes. The high incidence of avian tuberculosis in the RMP indicates that whooping cranes may be particularly susceptible to this disease. Together, these mortalities and the restricted