Distinguishing between sexes and species: bill size in Orange-fronted and Yellow-crowned Parakeets, Cyanoramphus auriceps

Abstract

Controversy surrounds the taxonomic status of the Orange-fronted Parakeet (previously known as Cyanoramphus malherbi), officially a colour morph of the more common Yellow-crowned Parakeet C. auriceps. We analyse bill length and bill width measurements from 60 museum specimens of Orange-fronted Parakeet and 44 museum specimens of Yellow-crowned Parakeet. Male Orange-fronted Parakeets have shorter bills than male Yellow-crowned Parakeets—the difference between sample means is 0.7–0.8 mm. Power calculations show that others have most likely not seen the difference as significant because of small sample sizes. Our analysis illustrates a useful statistical technique for identifying morphological differences between two species where both species are sexually dimorphic but the sex of each individual is uncertain.