How wry is a Wrybill?

Abstract

The laterally asymmetrical bill of New Zealand’s endemic Wrybill Anarhynchus frontalis is unique among birds and has inspired much debate regarding its evolution and functional significance. Despite this, only one previous study has attempted to quantify the range of individual variation in bill shape, but used a single metric of curvature (bill tip angle). Using standardized digital photographs of 40 live Wrybills, we explored a range of metrics of bill length and curvature to describe the variation in bill shape in greater detail. Like the previous study, we found no sexual dimorphism in bill shape, despite males being slightly longer-billed than females, and recorded similar variation in bill tip angle (16–23°). However, we found that this single metric under-represented overall variation in bill shape, due to significant differences in where curvature began and was most pronounced along the length of the bill. Principal component analysis indicated that at least three independent metrics were required to describe the shape variation among individuals. Subtle differences in bill shape could plausibly affect an individual’s relative success among the range of Wrybill foraging strategies observed in breeding and non-breeding habitats. Elucidating the potential behavioral and fitness consequences of this variation will require detailed foraging and demographic studies with individuals of known bill morphology.