Spatial size dimorphism in New Zealand's last endemic raptor, the Kārearea Falco novaeseelandiae, coincides with a narrow sea strait

Abstract

Although New Zealand's avifauna includes many unusual birds, species‐level diversity within lineages is typically low. There are, however, several instances where different allied forms are recognized in each of the two main islands. Among them is the Kārearea Falco novaeseelandiae, which is the only surviving endemic raptor species in New Zealand. Recent analysis confirms it to be a distinct lineage in the global radiation of this genus and most closely related to the Aplomado Falcon Falco femoralis of South America. We examined body size metrics and neutral genetic markers in Kārearea sampled across New Zealand to assess subspecific variation within the species. We found strong evidence using linear modelling and Bayesian clustering for two distinct sizes within Kārearea, in addition to the recognized sexual dimorphism. The boundary between the size clusters coincides closely with the Cook Strait, a narrow seaway between the two largest islands. However, analysis of mitochondrial sequence data and nuclear microsatellites showed no compelling partitioning at neutral loci. These data suggest adaptive change along a stepped environmental cline. Lineage splitting in Kārearea has either yet to become apparent in the distribution of neutral genetic variation and/or regional adaptation is proceeding despite gene flow.