Bird assemblages along a forest-degradation gradient in Neotropical seasonally dry forests

Abstract

Bird populations are declining globally, but the mechanisms underlying these declines vary among regions and ecosystems. Tropical birds may be particularly vulnerable because many species depend on specific habitat conditions and seasonal resource dynamics. However, how forest degradation is associated with bird assemblage change remains poorly understood in Neotropical seasonally dry forests (NSDFs). Here, we evaluated mist-netted understory and lower-midstory bird assemblages in 16 forest remnants of southern Ecuador along an integrated degradation gradient combining landscape-level anthropization and canopy height. Landscape anthropization was quantified from building density, road presence, farmland cover, and vegetation cover within 1-km² landscapes surrounding each site, whereas canopy height was used as a proxy of local forest structural condition. Because these two variables were highly correlated, we used their shared variation to represent an integrated degradation gradient and residual predictors to explore additional variation in canopy height and landscape anthropization. We analyzed α-diversity, β-diversity, and the richness and abundance of habitat-use, foraging-stratum, and diet generalists using Bayesian models fitted with and without a spatial smooth term. We recorded 486 captures belonging to 66 bird species. Captured species richness declined along the integrated degradation gradient in the non-spatial model, but this association became uncertain after accounting for spatial structure. In contrast, the canopy-height residual showed a clearer positive association with richness, suggesting that sites with taller canopies than expected for their level of landscape anthropization supported richer mist-netted assemblages. β-diversity responses were weak and partly contrary to expectations: dissimilarity metrics tended to decrease along the degradation gradient, suggesting possible compositional homogenization, although statistical support was limited. The clearest responses were observed for ecological strategies. Foraging-stratum and habitat-use generalists increased with degradation, particularly in abundance-based models, whereas diet generalists tended to decline. Overall, our findings provide exploratory evidence that forest degradation is associated with mist-netted bird assemblages through multiple, non-equivalent pathways. Conservation strategies in NSDFs should therefore consider both landscape-scale transformation and the maintenance or recovery of structurally complex forest remnants.