Morphometric Variation in Tundra Swans: Relationships among Sex and Age Classes

Abstract

Variation in 36 morphometric variables among 82 Tundra Swans (Cygnus columbianus) wintering in Humboldt County, California, was evaluated; 15 variables were analyzed in detail to identify patterns of morphometric variation and differences among sex-age classes. Adult males were the largest class, and males had a significant positive covariation between wing length and other external and internal morphological features. There was no such covariation among females. When 15 morphological variables were evaluated with a principal components analysis, seven components accounted for 74% of the observed variation. Only the first component, associated with external body size and heart weight, was aligned with sex and age differences. There was some sex-age variation associated with viscera weights, and to a lesser extent with tarsus size in relation to bill length. There was little sex-age variation among most bill measures, body fat measures, or spleen weight. Based on a discriminant function analysis, five of the 15 variables discriminated the morphologies of the sex-age groups. Our data suggest that sexual dimorphism is minor in immatures, but pronounced in adults. This increase of dimorphism with age occurred for most external and internal characters, but was not apparent for bill dimensions. We believe that Tundra Swans can be classified into three distinct groups on the basis of internal and external morphological features: (1) adult males have a relatively large body size but are similar in viscera weight and foot length to immature swans; (2) adult females are intermediate in body size, small in viscera weight, and smaller in foot length than adult and immature males; and (3) immature swans are relatively small-bodied, but similar in viscera weights and foot length measures to adult males.