Sex determination of birds is essential for ecological studies, but visually distinguishing the sex of monomorphic birds in the field is challenging. We investigated intersexual morphological differences in the Eurasian Tree Sparrow (Passer montanus saturatus), which is known as a sexually monomorphic species. We caught 198 adult Eurasian Tree Sparrows (104 males and 94 females), collected data on their morphological characteristics, and determined their sex using polymerase chain reaction to amplify sex-chromosome-linked CHD gene introns. Wing length was longer in males than in females, and badge sizes were significantly larger in males. Discriminant function analysis using badge size and wing length classified sex with an accuracy of 86.9%. In accordance with a previous paper, our study considers that the sex of Eurasian Tree Sparrows can be distinguished with reasonable accuracy by the badge size and wing length in wild populations.

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