Migration Dynamics of the Whooping Crane

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I r- 17, The compiled list of records for the 73 -year period 1912 -1984 includes 31 sightings total ing at least 120 whooping cranes that were actually observed on the Platte River. Of these, 26 sightings (84 percent) involving no less than 101 birds (84 percent) were observed within the critical habitat zone. Other sightings on the 1 ist were primarily of birds flying over the river, which is a semi- annual event for most whooping cranes in the Wood Buffalo-Aransas flock and does not verify stopover .use. It is noteworthy that among the records of whooping cranes sighted on the Platte River, markedly higher percentages of birds were observed within the critical habitat zone in the 38-year period 1912 -1949 (91 of 100 birds, or 91 percent) than in the subsequent 35 -year period 1950 -1984 (10 of 20 birds, or 50 percent) . This is likely influenced, to some extent, by locati onal bias in the pre -1950 records stemming from the fact that most of these earl ier sighti ngs were compil ed by Swenk and Brooki ng whose most active observer - correspondents were located in the Kearney area along the reach of the river included i n the critical habitat designation. Except for the 12 -year period 1920 -1931, graphical analysis of all whooping crane sightings on the Platte River from 1912 -1984 reflects a pattern of use consistent with the species' opportunistic (or nontraditional) strategy for selecting migratory stopover sites. Records for the period 1920 -1931 (when the migratory population consisted of only 39 to 24 birds) are anomalous in that they included nearly twice as many whooping cranes as were observed during the remaining 61 years of record. A closer examination of these records indicated that the disproportionately large number of whooping cranes observed from 1920 -1931 was most likely the result of identification errors, rather than any difference in the species migratory habits specific to this time period. The period from 1948 -1965 is also noteworthy in that the migrant population experienced a 2 -fold increase from 21 to 42 birds, although no whooping cranes were observed on the Platte River (and very few were sighted in Nebraska) over this 18 -year interval. The paucity of records during this time is 1 ikely due to a falloff in observation effort following the active interest in earl ier years motivated by Swenk, Brooking, and Allen. Consequently, it is probable that some undetected stopovers may have occurred on the river during these years. Since the early 1970's, extensive study and survey efforts have focused on the Platte River, including university- and government- sponsored research, as well as active monitoring by various state and federal agencies and private environmental groups. This active attention has undoubtedly reduced the likelihood of undetected whooping crane stopovers on the river in recent years. Nev erthel ess, from 1970 through 1984 - -a period when the migratory population increased from about 50 to 85 birds and sightings were increasing i n Nebraska - -only 5 confirmed sightings involving 15 birds were recorded on the Platte River. Viewed another way, the number of whooping cranes observed on the Platte River during the past 45 years, as determined from the Confirmed Sightings List (1940- 1984), is equal to 0.8 percent of the estimated 3,111 stopover opportuni- ties that have occurred (where one stopover opportunity occurs each time a migrant bird fl ies over the river). During the past 20 years (1965 - 1984), the vi .j