Morphometric sex determination in the endemic Yucatan Wren ( Campylorhynchus yucatanicus )

Sex determination in birds is vital for evolutionary biology and conservation efforts. However, sexing cryptic species can be challenging, especially outside the breeding season because of reduced dimorphic traits. This complicates conservation strategies and our grasp of ecological dynamics. This study aimed to develop a simple and reliable method to classify the Yucatan Wren (<em>Campylorhynchus yucatanicus</em>), an endemic and threatened species in the family Troglodytidae, using external structure measurements. A total of 141 individuals of the species were successfully captured using mist nets. We collected blood samples and measured various external structures. In the laboratory, we determined the sex of each individual with precision using the 1237L/1272H primer set, and the Classification and Regression Trees (CART) were employed to accurately identify their sex. Sexual dimorphism was pronounced, with males having greater body length and mass than females. Other sexually dimorphic variables included beak length, height from the nares, and base-measured beak length. The CART method was useful in distinguishing between the sexes, which will be very useful for future ecological studies on the conservation of <em>C. yucatanicus</em>.