Productivity, Sexual Dimorphism and Diet of North Island Kaka (Nestor Meridionalis Septentrionalis) on Kapiti Island

Abstract

Productivity in North Island Kaka (Nestor meridionalis septentrionalis) on Kapiti Island, varied markedly between two breeding seasons. As Kapiti Island lacks most of the introduced competitors now widespread on the main islands of New Zealand, this suggests that nesting in Kaka may be strongly limited by annual variation in plant food abundance even in the absence of introduced competitors. North Island Kaka display a disproportionate degree of sexual dimorphism in bill size, culmen length providing a reliable indicator of sex so long as the age-class of individuals is known. Similar sexual dimorphism in the Kea (N. notabilis), the sole extant congener of the Kaka, suggests that there is a phylogenetic basis for this condition. In view of the monogamous mating system of the Kaka and Kea, and the prolonged provisioning of females and young by males in both species, selection for enhanced male provisioning ability, rather than sexual selection, could maintain sexual bill dimorphism in these species. Kaka less than one year old can be distinguished from older birds by a pale periopthalmic ring, yellow cere and gape (birds < than five months old) and protruding rachides (feather quills) on the tips of the tail feathers (birds 3-6 months of age). The loss of juvenile characteristics prior to sexual maturity suggests that juvenile Kaka become socially independent of adults earlier than Kea, perhaps because of more readily obtainable food sources in their forest environment. On Kapiti Island Kaka take a diverse range of foods, including invertebrates, seeds, nectar and pollen, fruits, scale insects and sap. Most of the plant foods taken by Kaka on Kapiti are known to be eaten, or otherwise reduced in abundance, by Australian Brushtail Possums (Trichosurus vulpecula), a widespread potential competitor that has been eradicated from Kapiti. The "Niche Separation Hypothesis" (NSH), that sexual dimorphism in foraging-related anatomy can evolve as an adaptation to reduce intraspecific competition for food, has suffered from an inability to make predictions that cannot also be derived from the rival hypothesis of sexual selection. However, as sexual selection usually acts predominantly on males, greater female divergence in trophic anatomy is not readily explicable in terms of sexual selection theory. Unequivocal evidence for or against the NSH can, therefore, be provided by determining which sex has changed most with respect to the ancestral condition. Morphological comparison of the extinct Huia (Heteralocha acutirostris; Aves, Callaeidae) with its closest known relatives suggests that the pronounced sexual bill dimorphism of this species evolved through selection on female, rather than male bill form and therefore probably evolved as a means of reducing intersexual competition.