Sexual dimorphism in the North Island Kaka Nestor meridionalis septentrionalis: selection for enhanced male provisioning ability?

The North Island Kaka Nestor meridionalis septentrionalis, an endemic New Zealand parrot, displays a disproportionate degree of sexual dimorphism in bill‐size, males being 13.6% and 12.4% larger than females in culmen‐length and ‐depth respectively, but only 2.0% and 4.2% larger in tarsus‐length and cube root of body‐mass. Culmen‐length provides a reliable means of sexing Kakas if the age‐class of each bird is known; all adult and subadult males had exposed culmens ≥ 47 mm long, while those of females were less than this value. Juveniles with culmen‐lengths in excess of 44 mm were male. Similar sexual dimorphism in the Kea N. notabilis, the sole extant congener of the Kaka, suggests a phylogenetic basis for this condition. The monogamous mating system of the Kaka and Kea, together with the prolonged provisioning of females and young by males in both species, suggests that selection for enhanced male provisioning ability, rather than sexual selection, could be maintaining sexual bill dimorphism in these species.