Conserving nest trees used by cavity-nesting birds from endangered primary Atlantic forest to open farmland: Increased relevance of excavated cavities in large dead trees on farms

Abstract

Understanding nest-site selection is critical to conserving tree-cavity-nesting wildlife, but nest-sites may vary across landscapes. We examine variation in the characteristics of trees and cavities used by cavity-nesting birds from globally-threatened primary Atlantic Forest to open farmland with isolated trees. We predicted that nests would occur in the largest trees available, but that secondary cavity nesters (non-excavators) would increase their use of bird-excavated cavities and dead and exotic trees in open farmlands. We used a stratified case-control design and 20 random plots to assess variation in characteristics of trees and cavities (used and available) across gradients of canopy cover and distance to forest edge in subtropical Argentina. For secondary cavity nesters, nest cavities were more likely to occur in larger-diameter trees across all stand conditions, but more likely to occur in dead trees as canopy cover declined (i.e., in open farmland; n=123 nest trees). For primary excavators, nest cavities were more likely to occur in dead (vs. live) trees, with larger diameter, regardless of stand conditions (n = 54 nest trees). Available cavities declined from 4/ha in primary forest to 0.4/ha in open farmland. Cavities were increasingly of excavated origin in open farmland, including both available cavities and those used by secondary cavity nesters, which indicates that avian excavation may partly compensate for the loss of decay-formed cavities when large trees are cleared. As forest landscapes shift toward a predominance of agroecosystems, dead trees and primary cavity nesters may take on important roles in conserving cavity-nesting communities and their ecosystem functions. However, nest cavities declined in height and depth, and increased in entrance size toward open farmland, raising the possibility that birds increasingly use suboptimal cavities as forest cover declines.