Sexual Size Dimorphism and Mating Patterns

Abstract

Sexual size dimorphism (SSD) occurs in the vast majority of birds, although explanations for its existence are controversial. This chapter describes the results of a field study undertaken at the Delhi Zoo to establish SSD in the Painted Stork. A novel field study method was used, involving recording of copulating birds and obtaining measurements of tibia, tarsus, bill, body length, and body depth, on each individual in a pair from the images. 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 significant difference’s 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 those late in the nesting season. However, early male birds were recorded to have significantly larger tarsus, compared to later birds. The second aspect explored in this chapter is ­spatiotemporal patterns of nesting activity. In the Delhi Zoo, considerable variations were recorded in clutch size and nest initiation dates. Early birds, which colonized pond 2 first, exhibited a higher clutch size. The satellite colonies near the main colonies were always colonized last and had smaller clutch sizes.