. Bulletin. Natural history; Natural history. August, 1961 Bellrose et al.: Sex Ratios and Age Ratios 429 75 50 25. Hens Juveniles Hens Juveniles 1939 194! 1943 1945 1947 1949 1951 1953 1955 1959 Fig. 9.—Year-to-year changes in the hen percentage and in the juvenile percentage in mal- lards checked in hunters' bags in the Illinois River valley, 1939-1955 and 1959. Percentages have been adjusted so that the means for juveniles and hens are equal. comparison of the sex ratios in the sum- mer of one year with the sex ratios in the winter of the same or another year would be biased by the dispropo
. Bulletin. Natural history; Natural history. August, 1961 Bellrose et al.: Sex Ratios and Age Ratios 429 75 50 25. Hens Juveniles Hens Juveniles 1939 194! 1943 1945 1947 1949 1951 1953 1955 1959 Fig. 9.—Year-to-year changes in the hen percentage and in the juvenile percentage in mal- lards checked in hunters' bags in the Illinois River valley, 1939-1955 and 1959. Percentages have been adjusted so that the means for juveniles and hens are equal. comparison of the sex ratios in the sum- mer of one year with the sex ratios in the winter of the same or another year would be biased by the disproportionate hunting losses of juveniles, which have approxi- mately balanced sex ratios. As a means of measuring yearly pro- duction, sex ratios have certain basic ad- vantages over age ratios. For many spe- cies of ducks, such as the divers, age ratio data, derived from bag checks during the autumn, are difficult to obtain in ade- quate numbers. For the most important species, it is much easier to obtain large samples of sex ratio data from field ob- servations than to obtain large samples of age ratio data from bag inspections. Less skill is required to determine the sex of a duck in nuptial plumage than the age of a duck in any plumage. To test the validity of sex ratios as cri- teria of duck production, we have made a comparison, fig. 9, of sex ratios with age ratios, which are direct reflections of pro- duction. Both ratios were obtained from mallards killed by Illinois hunters in 1939- 1955 and 1959. We have also made, for several species of ducks, comparisons of sex ratios derived from observations on the breeding grounds, table 34, with the number of juveniles per hen killed in Illinois, table 62. As shown in fig. 9, the sex ratios (per cent hens) and the age ratios (per cent juveniles) obtained from mallards killed by Illinois hunters differed markedly in several years. The fluctuations in age ra- tios tended to be of greater magnitude than those in sex ratios; the peak
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