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Sexual dimorphism and sex determination in Black-necked Grebes Podiceps nigricollis: on the need to account for geographical variation in morphology and sex ratio |
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| Authors: | Pedro Sáez-Gómez Carlos Camacho Alberto Álvarez Nico Varo Cristina Ramo Andy J. Green |
| Affiliation: | 1. Department of Integrative Sciences, University of Huelva, Huelva, Spainpedro.saez@ciecema.uhu.es;3. Department of Evolutionary Ecology, Estación Biológica de Do?ana – CSIC, Seville, Spain;4. Estación Ornitológica Anastasio Senra (EOAS), Seville, Spain;5. Department of Wetland Ecology, Estación Biológica de Do?ana – CSIC, Seville, Spain |
| Abstract: | Capsule: The sex of Black-necked Grebes Podiceps nigricollis in Spain can be determined successfully using a locally derived discriminant function analysis (DFA) but a function derived from a North American population was not successful. Aims: To determine the extent of sexual size dimorphism in the Black-necked Grebe and develop discriminant functions based on morphometric data. We also assessed the accuracy of visual sexing by fieldworkers with different levels of experience and compared the discriminant function for European birds with one for North American birds, in order to determine the geographical specificity of the function. Methods: We used a data set of 370 Black-necked Grebes from a European population throughout the non-breeding seasons of 2008–11 in the Odiel saltmarshes (Spain), as well as published data from North America, to compare the accuracy of DFAs developed for European and North American populations when applied to the same population. Results: Males were significantly larger than females for all body measurements. The DFA with the highest success rate included one single linear dimension (head-bill length), showing an efficiency of 88.5% in the same sample used to build the function and up to 91.5% in independent samples of birds. The more experienced ringer showed higher accuracy in visual sexing than the less experienced one (84 vs. 70%), although self-consistency in sex assignment of a given individual was similar. The function developed for European birds performed poorly for American birds (79.1% sexed correctly) and vice versa (18.4% sexed correctly). Conclusion: Morphological and sex ratio differences between European and American subspecies both contribute to the discrepancy in the success rate. This illustrates the need for caution when applying discriminant functions developed in one bird population to individuals elsewhere. |
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