Predation is the major cause of avian nest failure, and an important source of natural selection on life history traits and reproductive behavior. However, little is known about the identity of nest predators in much of the world, including the Neotropics. To identify some of the nest predators exerting selection pressure on birds of the subtropical Atlantic forest, we present observations of animals depredating bird nests in Argentina and Paraguay. We recorded depredations (destruction or removal of eggs or nestlings) at 33 nests of 25 species of birds, confirming as predators ten species of birds (Squirrel Cuckoo Piaya cayana, White-eared Puffbird Nystalus chacuru, Toco Toucan Ramphastos toco, Red-breasted Toucan Ramphastos dicolorus, Saffron Toucanet Pteroglossus baillonii, Chestnut-eared Aracari Pteroglossus castanotis, Planalto Woodcreeper Dendrocolaptes platyrostris, White-throated Woodcreeper Xiphocolaptes albicollis, Buff-browed Foliage-gleaner Syndactyla rufosuperciliata, and Plush-crested Jay Cyanocorax chrysops) and two species of medium-sized mammals (White-eared Opossum Didelphis albiventris and Crab-eating Fox Cerdocyon thous), and inferring two additional mammal species (Black Capuchin Monkey Sapajus nigritus and Southern Tigrina Leopardus guttulus). Fifty-five percent of these nests were depredated by toucans or aracaris (Ramphastidae), which destroyed eggs and nestlings at cup-, closed- and cavity-nests. Red-breasted Toucans destroyed nests 1.6–22 m high, in habitats ranging from primary forest to a backyard. Mammals and jays depredated nests from ground-level to midstory, whereas woodcreepers and aracaris depredated nests from the midstory to canopy. We did not record snakes at any bird nests, in strong contrast to studies from other Neotropical forests. Further studies should examine trade-offs among nest concealment, physical protection, and parental defense behavior as means of reducing nest predation, and use camera traps to quantify nest predation rates by predator species.
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