We tested if Black-necked Grebe, a species in which both sexes undertake moult-migration, have an unbiased sex ratio at a moulting site in Europe, as previously found in North America and as was expected for a species with biparental care. For this we used a unique long-term dataset of 5821 grebes captured for ringing throughout the moulting seasons of 2006–2012 in the Odiel salt-marshes (SW Spain). The grebes were sexed and classified as adults (74%) or juveniles (26%). Birds ringed at Odiel were recovered over a wide area up to central Russia and south to the Canary Islands and Morocco. We report on a unique case of a strongly biased sex ratio in a moult-migrating bird species with biparental care, in which adult females were significantly more abundant than adult males in all 7 years (1.6–4.2 females per male). Biased sex ratios were not found among juveniles. Differences between North America and Europe in the sex ratios of adult Black-necked Grebes at moulting sites may be explained by the much larger American moulting sites, which would facilitate an unbiased sex ratio in North America, but not in Europe. Moulting sites in Europe may reach carrying capacity because of their smaller size, forcing the late migrating individuals (adult females and juveniles) to move longer distances to sites farther from breeding areas, such us the Odiel salt-marshes.
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Vol. 102 • No. 2