Sexual dimorphism may appear in various forms such as anatomical, plumage, or vocal features. Production of such variation can be costly. However, sexual dimorphism may provide long-term fitness benefits. The Sri Lanka Bush Warbler (Elaphrornis palliseri) is a sexually monomorphic and monotypic passerine endemic to the montane cloud forests of the island of Sri Lanka. The iris of this bird shows a marked variation and individuals have either red or ivory-cream colored irises. We studied the iris dichromatism of Sri Lanka Bush Warbler with respect to morphometric and sexual differences. We used PCR-based molecular sexing (n = 8 individuals) and multivariate statistics to analyze morphometric data (n = 20). We found that the iris dichromatism of Sri Lanka Bush Warbler is sex-specific: all females examined (n = 6) had ivory-cream irises and both males examined had bright-red irises, whereas none of the phenotypic traits examined were sexually dimorphic. Our results suggest that the sexual dichromatism observed in this dense understory–dwelling, cloud forest bird might play a central role as a short-distanced optical signal.
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