Analysis of External Morphological Characters in Determining Sexual Differences in the Bulbuls (Family: Pycnonotidae)

Most bulbul species (Family: Pycnonotidae) are monomorphic. Morphometric study of selected bulbul species was conducted using Sarawak Museum specimens (dry skin). The selected external morphological characters of the specimens were measured to determine differences between male and female birds (sexual dimorphism). Dimorphism index was used to calculate the dimorphism among selected species. Significant differences were observed in bill depth of straw-headed bulbul (Pycnonotus zeylanicus), wing length and tail length of yellow-vented bulbul (Pycnonotus goiavier) and also bill depth and tail length of ashy bulbul (Hemixos flavala). Dimorphism index shows that straw-headed bulbul has the highest index in bill length while the lowest is yellow-vented bulbul. Based on dimorphism index assessment, 78.1% of overall indexes show that males are larger than females based on external morphology.