| Resumo: | We studied the gut contents of 17 White-headed Ducks, 25 North American Ruddy Ducks and 26 hybrids between the two species collected from 14 Spanish wetlands. This is the most detailed study to date of Oxyura diet in the Palearctic region. Food items from at least 27 families of invertebrates and at least ten families of aquatic plants were identified. The method of collection of ducks and rapid digestion of soft-bodied invertebrates may have overestimated the importance of plant matter. However, animal foods were more important, constituting 73% of aggregate volume of gullet contents. In both duck species and their hybrids, benthic chironomid larvae and pupae were the most important food item, present in 69% of gullets and 75% of gizzards, and constituting 35% of aggregate volume and 26% of aggregate percentage in gullets. Angiosperm seeds were the next most important dietary component, and crustaceans (mainly Amphipoda, Cladocera and Isopoda) and green plant material were of secondary importance. No significant differences in diet were detected between duck species, sex or age classes. Birds sampled in the breeding season had more nematodes, but these may have been parasitic. Chironomids were less abundant in Ruddy Ducks collected from northern Spain outside of the range of the White-headed Duck.
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