Morphometric Differentiation in Island Avifauna: A Comparative Study

Abstract

Island avifauna provide exceptional natural experiments for studying morphometric differentiation driven by ecological release, founder effects, and resource-mediated selection in the absence of mainland competitors and predators. This study quantified morphometric differentiation across 24 passerine and near-passerine bird species representing 11 families sampled from six island groups (Baltic archipelago, Canary Islands, Azores, Madeira, Cape Verde, and Balearic Islands) and their nearest continental conspecific or congeneric populations (n = 3,842 individuals measured). Eighteen standard external measurements (wing length, tarsus, culmen, bill depth, bill width, tail length, and 12 additional variables) were recorded using digital callipers and wing ruler to 0.1 mm precision. Multivariate analyses (MANOVA, discriminant function analysis, principal component analysis) revealed significant morphometric differentiation between island and mainland populations in 21 of 24 species (MANOVA: all Wilks’ l < 0.62, p < 0.001). Island populations showed consistent trends of bill enlargement (mean bill depth increase: +8.4 ± 2.1%; bill width: +6.2 ± 1.8%) relative to mainland conspecifics, consistent with character release and dietary expansion. Wing length was significantly reduced in island populations of 16 species (mean -4.8 ± 1.4%), consistent with reduced migratory tendency and relaxed predation pressure. Tarsus length was significantly greater in island populations of 14 species (+5.6 ± 1.6%). DFA correctly reclassified island vs. mainland origin in 88.4 ± 4.2% of individuals across species. Morphometric differentiation was significantly correlated with island isolation distance (r = 0.72, p < 0.001) and island area (r = -0.61, p < 0.001). These findings support the ecological release hypothesis and provide a morphometric baseline for monitoring evolutionary responses to ongoing habitat change and invasive species pressure in island bird communities