The Ecomorphology of Sexual Dimorphism in the New Zealand Rifleman Acanthisitta chloris

Abstract

New Zealand Rifleman Acanthisitta chloris females are larger and browner than males (which are green dorsally), have a relatively larger hind claw and a longer, more decurved bill. Using three criteria for distinguishing sexual selection from ecological influences on sexual dimorphism, we suggest that sexual dimorphism in the Rifleman is maintained by ecological factors. When energy requirements were high (feeding fledglings), female and male Rifleman foraged separately. When energy requirements were low (non-breeding), females and males fed together. Plumage differences between the sexes were correlated with the different habitats in which Riflemen foraged when energy demands were high.