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.