Nest-site characteristics of Rufous-naped Wrens (Campylorhynchus rufinucha) in Acacia trees may serve to avoid vertebrate predators

The high rate of nest predation in tropical birds results in strong selection pressure. The Rufous-napedWren (Campylorhynchus rufinucha) nests in Bullhorn trees (Acacia) in the Mesoamerican dry forest. Ithas been proposed that bullhorns and their aggressive ants (Pseudomyrmex spp.) help to prevent nestpredation. I tested the hypothesis that these birds place their nests in particular acacia micro-habitats toavoid predation by vertebrates such as tufted capuchin monkeys (Cebus capucinus). I expected to findnests in locations that avoided the foraging behavior preferences of the monkeys. Along 6 km gravel roadin Palo Verde National Park, Guanacaste, Costa Rica, I found 52 Rufous-naped Wren nests. Theproportion of ant species in acacias with nests did not differ from the background proportion of ant speciesin acacias without nests, so birds did not prefer to nest in acacias with the most aggressive ant species.Acacia trees with wren nests were larger in diameter than control acacias without nests. I found more nestsin acacias that were clustered, which consisted of one to five acacias in a 3m radius plot around the acaciawith nest. However, the number of acacias with or without nests did not differ in their isolation from othernon-acacia trees. Finally, more nests were in unexposed sites within the tree, such as at the base ofbranches or in the topmost branches, compared with exposed sites such as outer branches. Therefore, theRufous-naped Wren seemed to place the nest in particular trees and places within them, in sites that maydecrease the probability access by a vertebrate such as C. capucinus to depredate their eggs and nestlings.Rufous-naped Wren nest site selection may also be influenced by other factors such as mechanical supportfor the weight of the nest, eggs, nestlings and adults.