Stable isotope analysis to examine possible diet-morphology relationships among Red-whiskered Bulbuls introduced on Réunion Island

A recent study on the introduced Red-whiskered Bulbul Pycnonotus jocosus on Reunion Island has highlighted the existence of rapid morphological divergence in bill size between the leeward and windward sides of the island. The hypothesis that diet could explain such geographical variation was explored by means of stable isotope analysis (C and N) on bird tissues. We found no evidence of geographical differences in δ15N and δ13C in muscle and feather tissues of birds collected on both sides of the island, despite morphological differences. Correlations between wing length, tail length and δ15N in tissues suggested that bigger birds could benefit from greater feeding opportunities on insects. However, stable isotope ratios in bird tissues marginally explained variation in bill size, indicating that diet specialisation may not be the major cause for morphological divergence among Red-whiskered Bulbuls on Reunion Island.