Nest site selection of secondary cavity-nesters in Quebracho Colorado Forest of the Humid Chaco: a preview

Secondary cavity-nesting birds select nest sites according to characteristics of cavities, nest trees and habitats. We examined the factors that influence nest site selection of secondary cavity-nesters in mature Schinopsis balansae forest of the humid Chaco. Using a case-control design, we contrasted different a priori hypotheses about protection against predators, body size and cavity durability. In 2016, we monitored 53 nest cavities used by 16 species in Chaco National Park, searched for an unused cavity for each nest cavity, and took 17 measurements at the scale of cavity, nest tree and habitat. Nests were located primarily in Schinopsis balansae (74%), at a height of 5.2 ± 2.4 m (mean ± SD) above ground, with entrance diameters of 6.3 ± 2.7 cm and depth of 40.9 ± 23.4 cm. Unused cavities were similar to nest cavities; they did not differ in the characteristics measured (t-tests; p > 0.1). We compared seven a priori generalized linear models but none explained nest site selection better than a null model. We interpret that: mature forests of Schinopsis balansae have abundant suitable cavities for nesting, and/or secondary cavity-nesters show high segregation in their nesting niches, so that there is no general pattern in nest site selection. During 2017-2019, we will increase sample size, identify cavities that are used/avoided in multiple years, and investigate how species traits (aggressiveness, migration and foraging site) influence nest site selection.