Female-biased sex ratio in moulting black-necked grebes podiceps nigricollis in Southern Spain

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.Peer Reviewe

Directional changes in sexual size dimorphism in shorebirds, gulls and alcids

sexual selection; size dimorphism; waders. The Charadrii (shorebirds, gulls and alcids) are one of the most diverse avian groups from the point of view of sexual size dimorphism, exhibiting extremes in both male-biased and female-biased dimorphism, as well as monomor-phism. In this study we use phylogenetic comparative analyses to investigate how size dimorphism has changed over evolutionary time, distinguishing between changes that have occurred in females and in males. Independent contrasts analyses show that both body mass and wing length have been more variable in males than in females. Directional analyses show that male-biased dimorphism has increased after inferred transitions towards more polygynous mating systems. There have been analogous increases in female-biased dimorphism after transitions towards more socially polyandrous mating systems. Changes in dimorphism in both directions are attributable to male body size changing more than female body size. We suggest that this might be because females are under stronger natural selection constraints related to fecundity. Taken together, our results suggest that the observed variation in dimorphism of Charadrii can be best explained by male body size responding more sensitively to variable sexual selection than female body size