Sexual Size Dimorphism and Assortative Mating in the Brown Noddy

We report the results of a biometric study of adult Brown Noddies (Anous stolidus) nesting within the Culebra National Wildlife Refuge, Puerto Rico in 1985-1988. The body mass of adults varied significantly between years, but linear measurements did not. Males were significantly larger than females in all body measurements. This difference also was reflected within pairs. A discriminant function using head-bill and wing length successfully predicted the sex of 90% of birds. Measurements of male and female partners indicated that they were not paired at random with respect to body mass and showed positive assortative mating for this character. The causes and consequences of this relationship are discussed.