Age Ratios of Sandhill Cranes

Abstract

Age ratios of sandhill cranes (Grus canadensis) at Last Mountain Lake, Saskatchewan, Canada, during fall migration were determined by observations of cranes on their roosts, feeding in fields, or flying between their roosts and feeding areas. Counts of flying flocks were found to give the least biased estimate of age ratios, and these data indicate an annual recruitment of 0.035 to the fall population in 1972 and 0.053 in 1973. J. WILDL. MANAGE. 38(2):234-242 Sandhill cranes were hunted for food and sport until 1918, when this species was placed under the protection of the Migratory Birds Convention Act. Partly because of crop depredations during fall migration, open seasons for hunting of sandhill cranes were established in parts of Alaska, Texas, and New Mexico in 1961, and in the provinces of Manitoba and Saskatchewan in 1964. Sandhill cranes are now hunted in the states of Alaska, North Dakota, South Dakota, Colorado, Oklahoma, Texas, and New Mexico and the provinces of Manitoba and Saskatchewan, and in recent years the bag limit has been increased and the areas open to hunting in most states and provinces has been enlarged. The total annual harvest for the United States and Canada in 1970 was estimated to be about 9800 cranes (Miller et al. 1972). Miller et al. (1972) used what seemed to be the best available data for different population parameters to construct a computer simulation model which would predict the consequences of various management alternatives for the migratory population of sandhill cranes in the Central Flyway. One of the assumptions in this model was that annual recruitment is 30 percent of the breeding population, or 18 percent of the total population. This value was taken as the best average of the proportion of juveniles in hunters' bags in New Mexico in 1961 (Boeker et al. 1961), Manitoba and Saskatchewan in 1964 and 1965 (Stephen et al. 1966), and Saskatchewan in 1966 and 1967 (Hatfield 1966, 1967, Canadian Wildlife Service unpublished report), the only seasons for which data were available; however, Stephen et al. (1966) showed that the percentage of juveniles in hunted samples may vary from as low as 11 percent to as high as 54 at different localities in different years, and Hatfield found that the percentage of juveniles in counts of cranes feeding in fields and flying to and from their roosts was approximately 4 and 6 percent when the proportion of juveniles in hunted samples was 19 and 20 percent respectively. Although Hatfield's unhunted samples were relatively small, these data and the wide variation in the proportion of juveniles in game bag checks suggest that the age ratio in a hunted sample is not a reliable indicator at annual recruitment. Miller et al. (1972) also accepted an estimate of 300,000 for the total size of the migratory population of sandhill cranes in the Central Flyway, based on ground surveys in Texas and New Mexico prior to each hunting season; however, it appears that the most reliable estimate is the spring survey in Nebraska, when all or nearly all of the cranes in the Central Flyway are concentrated along the Platte River (Bonde 1970, 1971, U. S. Fish and Wildlife Service un234 J. Wildl. Manage. 38 (2):1974 This content downloaded from 157.55.39.123 on Mon, 18 Jul 2016 05:21:59 UTC All use subject to http://about.jstor.org/terms CRANE AGE RATIOS * Miller and Hatfield 235 published report). The use of aerial photography and other improvements in the census technique have made these estimates particularly reliable in the last few years. The largest number of sandhill cranes counted during the spring survey in Nebraska was 207,500 in 1971. If we accept this estimate, rather than a population of 300,000 and if the crippling loss of 30 percent estimated by Miller et al. (1972) is correct, the total kill in the United States and Canada in 1970 was about 12,700 cranes, or approximately 6 percent of the population. Obviously, annual recruitment is a critical parameter in the management and protection of this species. If, for example, the annual recruitment is nearer 5 percent (Hatfield 1966, 1967) than 18 percent and the annual harvest is about 6 percent, this species is being endangered by excessive hunting. The purpose of this study was to develop an accurate and efficient method for obtaining age ratios in unhunted samples and to obtain an accurate estimate of annual recruitment in the hunted population of sandhill cranes in the Central Flyway. W. J. D. Stephen provided unpublished data on the weights and measurements of cranes collected at Last Mountain Lake and commented on the manuscript. J. P. Kelsall processed samples of adult and juvenile sandhill crane feathers for iron stain. This research was supported by a contract from the Canadian Wildlife Service.