Crested Caracaras (Caracara plancus) are diurnal, opportunistic raptors that feed on a great variety of prey, including carrion, nestlings, and eggs. In the coastal area of Chubut province, Argentina, they co-occur with White-headed Steamer-Ducks (Tachyeres leucocephalus), an endemic and vulnerable species. Here we report observations of Crested Caracaras on nests of White-headed Steamer-Ducks in the Interjurisdictional Marine Park in Northern San Jorge Gulf, Chubut, Argentina. We looked for steamerduck nests on islands and islets in the park from September to December in 2015 and 2016. We installed camera traps at 21 nests and operated them for a total of 166 camera-nights. We detected Crested Caracaras 27 times, at 15% and 37% of the duck nests in 2015 and 2016, respectively. We corroborated predation on eggs after visiting nests, although we were not able to see Caracaras directly eating eggs on pictures. The timing of Crested Caracara visits to duck nests corresponded with the recess pattern (i.e., periods of time when nest is unattended during incubation) of the White-headed Steamer-Ducks. Crested Caracaras are generally visual predators and they likely observe the steamerducks and take advantage of their incubation patterns. Our camera data contribute to improving our understanding of Crested Caracara ecology and potential limiting factors on White-headed Steamer-Duck nesting success.

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