Spring Sex and Age Ratios of Lesser Scaup and Ring-Necked Ducks in Minnesota

Abstract

Age and sex ratios were analyzed in a spring sample of 1,733 lesser scaup (Aythya affinis) and 447 ring-necked ducks (A. collaris) obtained in Minnesota in March, 1963. Determinations were made by dissection and internal examination. The bursa of Fabricius was used as an age indicator. By this technique 31.4 percent (141) of the ring-necked ducks and 17.1 percent (296) of the lesser scaup were judged to be immatures (yearlings). The sex ratios of the immatures did not deviate significantly from 1:1 although a slightly greater number of males were encountered in both species. The sex ratios among the adult ring-necked were 1.94 males per female, and in the adult lesser scaup 3.54 males per female. The adult sample therefore showed significant deviations from an even sex ratio. The number of immatures per adult female ring-necked was 1.36 and the number of immatures per adult female lesser scaup was 0.96. The index of bursal size is felt to be a useful and reliable measure for studying age composition in early spring populations of these species. Ancillary plumage and oviduct data support this conclusion and are discussed briefly. In December, 1962, and January, 1963, more than one million gallons of oil escaped onto the ice and snow on the Minnesota River at or near Mankato and Savage, Minnesota. In late March, 1963, the oil reached the Mississippi River. At this time waterfowl migration was well underway and included a great influx of lesser scaup and ring-necked ducks. Birds seeking food and resting places settled on the oil slicks. Peller (1963) described the resultant wildlife mortality. A total of 3,333 oil-soaked birds was picked up between Minneapolis and Redwing, Minnesota during the last week of March. Lesser scaup and ringneckeds constituted 65 percent and 16 percent respectively of the total observed kill. Little information is available concerning sex ratios of adults, sex ratios of immatures, and ratios of immatures to adults for lesser scaup and ring-necked ducks in the spring. The catastrophe provided an opportunity 'Present address: Zoology Department, University of South Dakota, Vermillion 57069. to acquire such data. The sample was also utilized for morphometric analyses (Anderson and Warner 1968a, b) and studies on lead shot incidence (Jessen et al., in press). R. Jessen and J. Lindmeir of the Minnesota Conservation Department are gratefully acknowledged for their efforts in collecting and making the birds available. We wish to thank D. Gates, E. LeFebvre, and D. Siniff for reading parts of the manuscript and for statistical advice. The assistance of Barbara Spross and S. P. Getman is gratefully acknowledged. Funds for transportation, storage and laboratory assistance were made available by the James Ford Bell Museum of Natural History, University of Minnesota. Additional storage space was made available through the Department of Poultry Husbandry, University of Minne-