Morphometric Correlates of Age and Breeding Status in American Coots

Abstract

Abstract I studied morphometric variation in 13 linear measurements from 228 American Coots (Fulica americana) collected in southern Manitoba. Univariate and multivariate techniques revealed differences in size and shape among adult coots that were 1, 2, and =2 yr old. In addition to the obvious differences in size between males and females, the morphometry of older birds differed from that of younger birds in two ways. First, older coots were of larger body size than younger coots of the same sex. Second, older coots had proportionately larger feet and claws relative to the size of their tarsi, and proportionately wider bills and heads relative to other head measurements, than did younger birds. Multivariate dispersion matrices within age/sex cohorts were less variable for older coots. In an analysis of 1-yr-old males, breeders did not differ from nonbreeders in overall body size, but breeders had relatively longer claws and wings than nonbreeders. Age-related differences in morphology may have relevance to the social structure of nesting coots, which involves highly aggressive territorial behavior. Part of the age-related variation in nesting phenology that has been documented elsewhere for coots may be a consequence of covariation in body size and shape.