Background Parrots (Psittacidae Family) are one of the most colorful groups of birds in the world, their colors produced both structurally and via unusual pigments (psittacofulvins). Most species are considered to be monogamous, and many have been viewed historically as sexually monomorphic and monochromatic. However, studies using morphometric analysis and spectrophotometric techniques have revealed sexual size dimorphism and also sexual plumage color dimorphism among some species. The Monk Parakeet (Myiopsitta monachus), a native parrot of South America, is an interesting species for the study of plumage coloration and size since it is considered sexually monochromatic and monomorphic. Furthermore, recent studies show that the Monk Parakeet has extra-bond paternity behavior and even breeding trios, which suggests that sexual selection may play an important role in this species, and that it might have sexually dimorphic plumage (albeit imperceptible by humans) and be dimorphic in size.

Methods For the determination of plumage color we used spectrophotometry in the range of avian vision (300?700 nm) and performed a morphological analysis.

Results Our spectrophotometric results indicate that the Monk Parakeet shows subtle sexual plumage color dimorphism in three (crown, nape and wing) out of twelve body regions. Similarly, our morphometric analysis showed that there are subtle sex differences in body size (bill and weight).

Conclusions Although the Monk Parakeet shows extra-bond paternity and breeding trio behaviors which could increase sexual dimorphism, these behaviors occur among highly related individuals; perhaps the high rate of inbreeding is responsible for the attenuation of sexual plumage color dimorphism and sex differences in body size observed.