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often difficult to confirm (even experts can have trouble distinguishing sand- hills and whoopers from a distance). The recently established Cooperative Whooping Crane Tracking Project, an attempt to gather.data in a more systematic fashion, may remedy these shortcomings. However, 3 years of probable and con- firmed sightings (1976-78) seem to support the earlier data, with 20% of all sightings in Nebraska (Table IIIc). This project cannot determine the behavior of individuals, and so much remains to be learned about whooper migration pat- terns. Next year, the cranes will be fitted with telemetric devices, and many gaps in the current picture will be filled in. 6. An indequate supply of roosting sites in the Central Flyway will reduce the spec ies chances of survivaL Not knowing how the whooping crane will respond to changes in its migratory habitat makes it difficult to estnnate the effects of such changes on the crane's mortality. It is nontheless in- structive to look at the existing data on crane population. In 1938, when Aransas National Wildlife Refuge was established, the Aransas flock numbered only 18 individuals. By 1976 it had grown to 69 birds (see Fig. 1). In other words, the population of the Aransas Flock nearly quadrupled during precisely , the period when the channel width in the critical habitat along the Platte , decreased by 50%. These data suggest that the loss of habitat along the crane's migration route was not a limiting factor. .C. i "A It is not completely understood why the whooper population has improved V"~~~;0V~0 dramatically, although when average hatching and death rates by decade are ~\l'1'~)v\ computed, some suggestive trends emerge (see the note to Table IV for defini- \ \:l"~ tions of hatching and death rates). As shown in Table IV, the death rate for vfJ',' the Aransas population declined from 0.169 in the decade 1938-47 to 0.065 in the interval 1968-75. This decline must be the principal reason for the dram- atic population increase over this period and to some extent is the result of the additional protection provided against hunters by the Aransas sanctuary. But though the wildlife refuge opened in 1938, the population in 1955 was the same as in 1940. Except for brief periods in the early 1960s and early 1970s, the population increase since 1954 has been fairly steady. Over the same period, however, the birth rate also declined considerably--from 0.206 to 0.111. Why this should have happened has puzzled biologists at the Fish and wildlife Service. 42