Sex Determination and Sexual Size Dimorphism in the Red-billed Tropicbird (Phaethon aethereus) and White-tailed Tropicbird (P. lepturus)

Abstract

Abstract. Tropicbirds (order Phaethontiformes) are a well-studied seabird taxon, but there have been no published studies on sexual size dimorphism. Here, we investigated sexual size dimorphism in the Red-billed Tropicbird (Phaethon aethereus) and the White-tailed Tropicbird (P. lepturus), and evaluated the applicability of using morphometrics and discriminant functions to sex birds in the field. We sampled Red-billed Tropicbirds (n = 160) and White-tailed Tropicbirds (n = 57) in the Atlantic Ocean off the coast of Brazil. Each individual had seven morphometric traits measured and blood sampled for molecular sex determination. Discriminant functions were generated using generalized linear models (GLM). Red-billed Tropicbirds had significant male-biased intersexual differences in bill length, wing chord, nostril-to-bill-tip, and head-plus-bill, and the best GLM included bill length and wing chord. White-tailed Tropicbirds were characterized by significant intersexual differences for wing chord, with female...