Sex Discrimination Based on External Morphological Measurements in Brown-eared Bulbul Hypsipetes amaurotis

Abstract

Ten external morphological variables were obtained from four different kinds of specimens of Brown-eared Bulbul Hypsipetes amaurotis which differed both in capture site and in condition of storage. Almost all the variables were larger in males than in females, showing a pronounced sexual size dimorphism. The specimens kept as study skins differed in both means and variances of almost all of the variables from fresh or frozen specimens.
A discriminant function analysis was done for the specimens which had been collected from the same site in the same season. As a result, a discriminant function incorporating the lengths of tarsus, bill from nostril, total length, wing span, total head and width of bill at nostril could discriminate sex correctly at the probability of wrong discrimination of 0.021. Another two discriminant functions for more general situations were derived from an analysis of specimens collected from various sites, incorporating lengths of tail, tarsus, bill from nostril, wing span and total head, or the lengths of natural wing, tail, tarsus, exposed culmen and total head. We can use these functions for discriminating sex with an accuracy of 95%.
These functions correctly classified sex of only 74% of study skins, showing that they could not be reliably applied to these specimens.