Age Determination and Molt in the Boat-Tailed Grackle

Abstract

1957). This study concerns criteria of age in the Boat-tailed Grackle (Cassidix mexicanus prosopidicola) derived from examination of specimens from central Texas. Special attention is given to criteria by which first-year and adult birds can be distinguished, in order to compare the annual reproductive cycles of these two age groups (Selander and Hauser, MS). The investigation is also designed to serve as groundwork for the author's comparative studies of behavior and phylogeny in the grackles. The present study is based on 328 males and 165 females collected over a 16-month period in 1956 and 1957, within a ten-mile radius of Austin, Travis County, Texas. Specimens were processed as follows: The left testis or ovary was removed and placed in Bouin's fixative in preparation for histologic studies; body weight was recorded (within two hours after death); extent of molt, if any, was recorded in detail for each feather tract and its regions; types and numbers of juvenal feathers retained through the postjuvenal molt by first-year individuals were noted; extent of skull ossification was determined; iris color was recorded; relative amount of body fat was noted; and measurements of wing, tail, bill, tarsus, and middle toe were taken. Approximately 10 per cent of the specimens were prepared as study skins or skeletons and deposited in the Texas Natural History Collection. Age determination of males presents relatively few problems. First-year males are duller and less glossy or iridescent than adult males and generally can be recognized at a glance (fig. 2). The duller color, in combination with a shorter and narrower tail, is generally so conspicuous that first-year birds are recognizable in the field. Care must be exercised in individual cases, however, because some first-year males are nearly as iridescent and dark as adults, particularly after the prenuptial molt, and have adultlike tails. Age determination of females is a much more difficult problem and cannot be accomplished with certainty on the basis of plumage color alone. In worn plumage, color is of little value in age determination. In studying the race C. m. monsoni, Phillips (1950) had only limited success in aging female museum specimens, and he stressed the need in taxonomic studies for females of known age. The need to establish criteria by which first-year and adult females can be identified with confidence and to test the validity of plumage color as an age criterion in males has necessitated a detailed investigation of molt. Characters other than color that were examined for use in determining age are feather shape and degree of wear, retention of juvenal feathers through the postjuvenal molt, iris coloration, degree of skull ossification or double layering, and size. Using a combination of characters, it has been possible to age all specimens collected. Some characters are of value not only in Cassidix but in related genera of icterids as well. MOLTS AND PLUMAGES