Are Insular Populations of the Philippine Falconet (Microhierax erythrogenys) Steps in a Cline?

. Founder effects, new environments, and competition often produce changes in species colonizing islands, although the resulting endemism sometimes requires molecular identification. One method to identify fruitful areas for more detailed genetic study is through comparative morphological analyses. We measured 210 museum specimens to evaluate the potential morphological consequences of colonization across the Philippine archipelago by the Philippine Falconet (Microhierax erythrogenys). Measurements of both males and females differed clearly from island to island. Univariate and multivariate analysis of characteristics showed a latitudinal gradient, with the bill, wing, and tail of southern birds being larger than those of northern birds, forming the pattern of a stepped cline across a succession of islands. The stepped gradient in morphology and extensive differences between islands we observed provide evidence for multiple perspectives on phylogeny, including concordance with aggregate complexes expected on the basis of sea-level fluctuations. However, calculation of diagnosability indices did not support subspecific designations. Sex-specific dominance and dispersal patterns may explain this unusual south-to-north stepped cline, and they also provide a useful format for understanding biogeographical patterns by island. Finally, these morphological data suggest a potentially fruitful area for future genetic studies.