Three species of Brazilian thrushes replace one another ecologlcally along the primary and secondary forests of the Eastern Amazonian Region, Turdus T fumigatus and T. leucomelas. These three species are monochromatic, i. e., me and female have similar plumages. Whether these species are monomorphic (i.e., if males and females are of similar size) or not has not been previously investigated. Studies in Mexican forests indicated that some monochromatic birds from the Neotropical Region are in fact cryptically dimorphic, i. e., males and females differ statistically in size when suitable statistic techniques are appiled. This work has three main objectives: (a) to evaluate the pattern of sexual dimorphism in size in T. albicollis phaeopygus, T. fumigatus fumigatus and T. ieucomelas albiventer, (b) to contribute to the study of the sexual dimorphism in size of Neotropical monochromatic birds, and (c) to provide subsidies for evolutionary and ecological studies on the genus Turdus, and also on the family Turdidae as a whole. The working hypothesis here was the three species of Turdus studied would be cryptically dimorphic in a pattern similar to the passeriform forest birds previously studied in the Mexican forests. Of the three species studied, two were found to be monomorphic (T. f fumigatus and T. a. phaeopygus) and one cryptically dimorphic (T. 1. albiventer). In the only cryptically dimorphic species, males differ significantly from females in the length of the wing, tad, tarsus and fourth toe claw. However, a reliable sexual identification cannot be performed from the discriminant linear function obtained. The reason the three species of Turdus are monomorphic or cryptically dimorphic may be associated with their pre-reproductive behavior. During the mating season. vocalization seems to be more important to attract females and for territorial defense than plumage or size. Thus, there is a strong selective pressure for vocalization of males and weak or non-existent pressure for body size. It is suggested that more research for the evaluation of sexual dimorphism in other species of Turdus and. a phylogenetic analysis of this large genus are indispensable in clarifying the evolution of patterns of sexual dimorphism in thrushes.
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