Sexual size dimorphism (SSD) occurs in the vast majority of birds, although explanations for its existence are controversial. We studied SSD in the Painted Stork (Mycteria leucocephala), a large wading bird, by recording copulating birds on a video camera and obtaining measurements of tibia, tarsus, bill, body length and body depth, on each individual in a pair. Field studies were conducted in the natural heronries of the Delhi Zoo in North India. For quantifying the dimensions of various body parts we improvised upon a microscopic software (MOTIC IMAGES) and developed a protocol for obtaining the dimensions of body parts in metric units. Males were larger than the females in all of the 100 pairs recorded during the study period. A PCA performed on the available data set of male and female independently showed a significant difference in the first PC scores, which were highly correlated with body length in both sexes. A tendency towards positive assortative mating was observed by plotting the male and female values of different variables. There were no significant differences in the median body lengths of individuals recorded copulating early and late in the nesting season. However, early male birds were recorded to have significantly larger tarsus, compared to later birds. The methodology used in this field study has wide applicability for studying the SSD and mating patterns in various groups of waterbirds.
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Vol. 29 • No. 4