Morphometric methods of sex differentiation may be cheap, simple, quick, and reliable alternatives to molecular approaches. However, there are still important uncertainties regarding the use of morphometric methods in birds, particularly regarding their applicability to different populations and environmental conditions. Between 2004 and 2019, we sampled 245 Bonelli´s Eagle Aquila fasciata nestlings, 197 from nests in Catalonia (northeastern Spain) and Andalusia (southeastern Spain) and 56 that were partially or totally raised in captivity. Our objective was to develop morphometry-based sexing methods for Bonelli’s Eagle nestlings that can be applied in situ to different subpopulations and growth conditions. We recorded up to nine measures related to nestling body mass, bill, tarsus, and claw length. Tail and wing length were used to control for the age of nestlings, and all individuals were sexed genetically. We found important morphometric differences between the two natural subpopulations. According to our discriminant analyses, body mass and claw length were the most discriminant variables, both in wild and captivity conditions. Differences between sexes were more prominent during the flight feathers’ growing period. Feeding ad libitum in captivity conditions led to larger nestlings and reduced between-sexes differences. We provide different models for sexing Bonelli’s Eagle nestlings that can be applied according to the studied geographic location, growth conditions (wild versus captive), and balance between accuracy and simplicity. Overall, our findings highlight that extrapolating sex discriminant functions to different subpopulations and growth conditions may be risky.

性别分化的形态测量方法可能是分子方法的廉价、简单、快速和可靠的替代方案。然而，在鸟类中使用形态测量方法仍然存在重要的不确定性，特别是关于它们对不同种群和环境条件的适用性。在 2004 年至 2019 年期间，我们采样了 245 只 Bonelli 的 Eagle Aquila fasciata雏鸟，197 只来自加泰罗尼亚（西班牙东北部）和安达卢西亚（西班牙东南部）的巢穴，56 只部分或全部被圈养。我们的目标是为 Bonelli 的鹰雏鸟开发基于形态测量的性别鉴定方法，该方法可以原位应用于不同的亚群和生长条件。我们记录了多达九个与雏鸟体重、喙、跗骨和爪长相关的测量值。尾巴和翅膀的长度被用来控制雏鸟的年龄，并且所有个体都进行了基因性别鉴定。我们发现了两个自然亚群之间的重要形态学差异。根据我们的判别分析，在野生和圈养条件下，体重和爪长是最具判别性的变量。在飞羽的生长期，两性之间的差异更为突出。在圈养条件下随意喂养会导致更大的雏鸟并减少两性之间的差异。我们提供了不同的模型来对 Bonelli 的鹰雏鸟进行性别鉴定，这些模型可以根据所研究的地理位置、生长条件（野生与圈养）以及准确性和简单性之间的平衡来应用。总体而言，我们的研究结果强调，将性别歧视功能外推到不同的亚群和生长条件可能是有风险的。