How Forest Structure and Microclimate Influence Long-Term Avian Diversity

Agnes Marian Hartong — Master's thesis, Pontificia Universidad Católica de Chile, January 2026

Temperate rainforests of southern Chile form part of a globally significant biodiversity hotspot, yet their avifauna remains poorly documented in long-term monitoring programs. This thesis examines how forest structural complexity and short-term microclimatic conditions shape bird species richness, species-level densities, and functional guild patterns across old-growth and second-growth Andean forests in the La Araucanía Region.

The study draws on 2,025 standardized point-count surveys conducted between 2010 and 2025 across 21 forest sites spanning an elevational gradient of 500–1,400 m a.s.l. Old-growth forests (>200 years) were characterized by multilayered canopies, abundant deadwood, and dense Chusquea understory, while second-growth stands (40–100 years, most at 80–90 years of recovery) exhibited simpler structure dominated by broadleaf species such as Nothofagus obliqua and Laureliopsis philippiana. Statistical analyses used hierarchical generalized linear mixed-effects models (GLMMs), incorporating distance sampling, random effects for site and season, and zero-inflated negative binomial models for detectability.

Contrary to expectations, forest type had no significant effect on local species richness. A total of 55 species were recorded, 41 shared between both forest types. When microclimatic predictors were added to the models, the apparent richness difference between forest types disappeared, indicating that weather-driven variation in detectability had masked any structural signal. The primary driver of detectability was wind speed, which negatively affected the probability of detecting birds throughout the survey window. Survey start time also strongly influenced detections, with bird activity declining steadily through the morning, supporting the recommendation to restrict surveys to 06:00–09:00 h. Temperature had a weak but positive effect on detectability.

At the species and guild level, forest structure did exert measurable effects. Vertical profile generalists and non-cavity nesters occurred at significantly lower densities in old-growth forests, reflecting their affinity for the denser shrub layers and simplified vegetation structure of second-growth stands. By contrast, large-tree users and understory specialists tended toward higher densities in old-growth sites, consistent with their dependence on standing deadwood, tree cavities, and vertical habitat complexity — though these trends did not reach statistical significance. Primary and secondary cavity nesters showed a non-significant tendency toward greater abundance in old-growth forests, aligned with the higher availability of coarse woody structures there. Foraging guilds, foraging substrate groups, and migratory guilds showed no significant differences between forest types.

A consistent finding across all guild dimensions was high interspecific variability, with species within the same guild often responding in contrasting directions. This heterogeneity diluted guild-level patterns and underscores the risk of over-generalizing from functional group means alone.

The study concludes that old-growth and second-growth forests make complementary contributions to regional bird diversity: old-growth patches sustain cavity-dependent specialists and structurally sensitive species, while advanced second-growth forests support open-nesting generalists during successional recovery. Effective conservation of Andean temperate avifauna therefore requires maintaining landscape-scale structural heterogeneity rather than prioritizing a single forest type. The findings also emphasize the importance of integrating climatic covariates into long-term monitoring design, particularly wind conditions, to avoid bias in community assessments.

Tesis Agnes Hartong 2026.

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