Nest predation is the primary cause of nest failure in passerines. In order to contribute to our understanding of how nest predators shape avian nesting ecology and life history traits, we report nest predator identity and nest predation rates for 3 species of passerines in the central Andes of south temperate Argentina. We used video cameras and opportunistic observations with photographic documentation to identify nest predators of Grass Wrens (Cistothorus platensis) breeding in a riparian grassland, as well as House Wrens (Troglodytes aedon) and House Sparrows (Passer domesticus) breeding in nest boxes in a tree plantation. From 13 nest predation events we were able to identify 3 nest predator species: mousehole snake (Philodryas trilineata), South American gray fox (Lycalopex griseus), and American Kestrel (Falco sparverius). Field observations also suggested fire ants (genera Solenopsis) as a possible nest predator. Mousehole snakes were identified at both grassland and forest plantation, representing 76.9% of the identified predation events. House Sparrows had the highest nest predation rate (43.1%), followed by Grass Wrens (30.8%) and House Wrens (29.3%). Egg predation was more frequent for House Wrens (64.2%) and House Sparrows (50.0%) than for Grass Wrens (22.1%). In contrast, nestling predation was considerably higher for Grass Wrens (77.9%) than for House Wrens and House Sparrows (34.7% and 50.0%, respectively). Knowledge of the nest predator community and information of nest predation rates of different species in temperate South America will contribute to understand nest predation effects on patterns and processes of nesting success, life history traits, and future management decisions in this region.

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