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number of whooping cranes observed on the Platte has been equivalent to about 1 <br />percent of the corresponding stopover opportunities. Confirmed records for <br />consecutive 5 -year intervals from 1965 -1984 reflect similar stopover use over <br />time (expressed as a percentage of opportunities), even though survey efforts <br />along the Platte have increased and the size of the Wood Buffalo-Aransas <br />population has more than doubled during this 20 -year period. <br />Comparisons with 0 hPr Stopover Aas in Nebraska (Section 3.4): Although it <br />is not known whether the Rainwater Basin and Niobrara River in Nebraska have <br />received as much observation effort as the Platte River, confirmed sightings <br />from 1950 -1984 indicate that more whooping cranes have been observed in the <br />standing -water wetlands of the Rainwater Basin than on or near either of the <br />two rivers. This finding appears to be consistent with the species' tendency <br />to use small ponds, marshes, and other standing -water wetlands along the flyway <br />as documented during radio-tracking surveys. Moreover, over this 35 -year <br />period, more whooping cranes are known to have stopped on the Niobrara River <br />than on the Platte. <br />Based on an evaluation of confirmed sightings for consecutive 5 -year periods <br />from 1950 -1984, the number of migrant whooping cranes observed in each of these <br />three habitat areas has generally increased over time. The greater number of <br />birds sighted in recent years is 1 i kely attributable to the increasing size of <br />the migrant Population and /or increasing observati on effort. <br />Implications (Section 3.5) : In light of recent information., and given the <br />small size of the whooping crane population throughout recorded history, it is <br />improbable that habitat alterations along the Platte River or other sites along <br />the migration corridor have been limiting to the survival and recovery of the <br />species. This conclusion is supported by the following lines of evidence: <br />1) The 70 -mil a reach of the Platte River between Overton and Grand Island, <br />Nebraska (which includes much of the designated critical habitat zone) <br />currently serves as a stagi ng area for a thi rd -of -a -mil l ion sandhil l <br />cranes - -a congeneric species generally considered to have roosting habitat <br />requirements similar to those of the whooping crane. From a comparative <br />standpoint, the present population of whooping cranes in the Wood <br />Buffalo-Aransas flock (approximately 85 birds as of January 1985) is <br />equivalent to only 0.02 percent of the sandhil l cranes in the Platte River <br />staging population, or a ratio of 3,918 sandhill cranes for every whooping <br />crane. <br />2) Sandhil l cranes stage along the Platte River on a traditional basis, <br />remaining for approximately 30 days. Whooping cranes, in contrast, are not <br />dependent on specific habitat along the flyway. Instead, they select <br />stopover sites on a nontraditional basis and typically use them for short <br />periods (primarily overnight stays of about 12 to 16 hours). Consistent <br />with this strategy of use, the number of whooping cranes known to have <br />stopped on the Platte River over the past 45 years (1940 -1984) represents <br />less than 1 percent of the stopover opportunities (less than 2 percent if <br />the assumption is made that twice as many birds stopped on the river as were <br />sighted). <br />Vii <br />