Effects of Age on Reproduction in American Coots

Abstract

Relationships between age and breeding biology of American coots (Fulica americana) were studied during 1972-74 in northwestern Iowa. Coots 3 years old and older began nesting about the same date, whereas younger birds nested later within a season. Clutch size increased with age, but only 1and 2-year-old birds showed seasonal and annual variations in mean clutch size. Oneand 2-year-old coots hatched proportionally fewer eggs within a clutch than did older birds. Yearling coots laid smaller eggs and fledged young of lighter weight than did older birds. Fledging success increased with age. Estimated figures from 1 study area indicated that of 471 adults, 57% were 1 year old, 25% were 2 years old, 14% were 3 years old, and 4% were 4 years old or older. Fifty-eight percent of the yearlings nested, and 96% of the 2-year-old coots and all older birds did so. J. WILDL. MANAGE. 44(1):183-189 Age is an important factor affecting reproduction. It may directly influence nesting chronology of birds (Grice and Rogers 1965, Coulter and Miller 1968), clutch size (Brakhage 1965, Morse et al. 1969), and fledging success (Trauger 1971, Ainley and Schlatter 1972). Knowledge of age-related parameters of reproduction is essential for proper understanding of the population dynamics and productivity of a species (Lack 1966:245). The American coot is amenable to a study of age-specific breeding biology. Gullion (1952), Burton (1959), and Crawford (1978) described techniques for age determination, and Fredrickson (1970) noted that nesting chronology and clutch size may be influenced by age. Gullion (1952) and Fredrickson (1968) determined the relationships between body size and age, but little information exists on the relationships between age and breeding biology. The purpose of this study was to relate selected parameters of breeding to the age of adults. I thank M. W. Weller for advice and encouragement throughout the study. K. Abraham, S.-A. Bengtson, L. H. Fredrickson, D. G. Raveling, M. W. Weller, and 2 anonymous reviewers read various drafts of the manuscript. G. W. Gullion provided valuable discussions on breeding biology and techniques of age determination, and L. H. Fredrickson and D. L. Trauger were helpful in formulating early ideas for the study. The Iowa Conservation Commission provided logistical