The Allometry of Morphology in Flora and Fauna of New Zealand and Offshore Islands

Flight is the primary form of locomotion for many avian species and is enabled by allometric scaling of morphological features such as wingspan, flight muscle size, and bone tensile strength. Contrary to this, the evolution of flightlessness in birds displays a selection towards an increase in body size with a reduction in flight associated features. The aim of this chapter is to explore the Loss of Dispersibility hypothesis as a cause for flightlessness in island birds, with consideration of the Island Rule and the Size-Constraint hypothesis. With island species paired with closest mainland relatives, comparative analyses were conducted comparing the change in wing loading ratios, wing lengths, and mass. With paired t-tests and Major Axis linear regression modelling, the hypotheses of isometric or allometric scaling in each of the features were tested. An increase in wing-loading ratio was apparent for many island species, as well as an increase in both mass and wing length. However, the rate of increase between mass and wing length is disproportionate, with mass increasing at a greater rate than wing length. These trends reject the Loss of Dispersibility hypothesis in support of the Size-Constraint hypothesis while providing little evidence for the Island Rule.