The relative investment of females and males into parental care might depend on the population's adult sex-ratio. For example, all else being equal, males should be the more caring sex if the sex-ratio is male biased. Whether such outcomes are evolutionary fixed (i.e. related to the species' typical sex-ratio) or whether they arise through flexible responses of individuals to the current population sex-ratio remains unclear. Nevertheless, a flexible response might be limited by the evolutionary history of the species, because one sex may have lost the ability to care or because a single parent cannot successfully raise the brood. Here, we demonstrate that after the disappearance of one parent, individuals from 8 out of 15 biparentally incubating shorebird species were able to incubate uniparentally for 1-19 days (median = 3, N = 69). Moreover, their daily incubation rhythm often resembled that of obligatory uniparental shorebird species. Although it has been suggested that in some biparental shorebirds females desert their brood after hatching, we found both sexes incubating uniparentally. Strikingly, in 27% of uniparentally incubated clutches - from 5 species - we documented successful hatching. Our data thus reveal the potential for a flexible switch from biparental to uniparental care.

Department of Behavioural Ecology and Evolutionary Genetics Max Planck Institute for Ornithology Seewiesen Germany

Department of Biology and Centre for Environmental and Marine Studies University of Aveiro Aveiro Portugal

Department of Ecology Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic

Department of Wildlife Diseases Leibniz Institute for Zoo and Wildlife Research Berlin Germany

Department of Zoology Faculty of Environmental Sciences Charles University Prague Prague Czech Republic

Equipe Ecologie Evolution UMR 6282 Biogéosciences Université de Bourgogne Franche Comté 6 Bd Gabriel Dijon 21000 France

Groupe de Recherche en Ecologie Arctique 16 Rue de Vernot Francheville 21440 France

Poelweg 12 1778 KB Westerland The Netherlands

South Iceland Research Centre University of Iceland Selfoss Iceland

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