. Bulletin. Natural history; Natural history. August, 1961 Bellrose et al.: Sex Ratios and Age Ratios 449 acres. For example, mallards checked at Stuttgart, Arkansas, were shot on at least 20 different swamp or reservoir tracts scat- tered over an area having a 25-mile radius. Most of the mallards checked in the Illi- nois River valley were bagged at 10 clubs distributed over a linear distance of 100 miles. Only a slight difference in the number of juvenile mallards per adult between populations of the upper and lower sec- tions of the Illinois River valley was found in 1939, table 54. This di
. Bulletin. Natural history; Natural history. August, 1961 Bellrose et al.: Sex Ratios and Age Ratios 449 acres. For example, mallards checked at Stuttgart, Arkansas, were shot on at least 20 different swamp or reservoir tracts scat- tered over an area having a 25-mile radius. Most of the mallards checked in the Illi- nois River valley were bagged at 10 clubs distributed over a linear distance of 100 miles. Only a slight difference in the number of juvenile mallards per adult between populations of the upper and lower sec- tions of the Illinois River valley was found in 1939, table 54. This difference was not significant at the 90 per cent level (X2=, 1 ). Much greater differences in number of juveniles per adult for several species of ducks were found between populations of two marshes 25 miles apart and adjacent to the Great Salt Lake, Utah, in 1947- 1949, tables 55-57. The probability that the differences were the result of chance is shown in tables 55—57. There was a sta- tistically significant difference betrween age ratios in the two areas in 11 of 18 tests. These data suggest that the age compo- sition of migrating flocks differs and that fortuitous circumstances result in flocks especially numerous in birds of one age class or the other in a particular marsh. Where only one waterfowl area in a re- gion has been sampled, as Winous Point in Ohio or Pointe Mouillee in Michigan, the age ratios derived may or may not reflect those for the entire region. For species other than the mallard, re- gional data are not adequate to permit evaluation of the age ratios derived in any one area. For each of these species, we have compiled data from as many areas as possible in the Mississippi Flyway on the assumption that data for the total flyway represent the species better than the data from any one area and reflect year-to-year changes in the age composition of the pop- ulation. Factors Affecting Age Ratios Age ratios can be used for appraising the productivity
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