Forest Decline Differentially Affects Bird Abundance Across Functional Groups: A Meta‐Analysis

ABSTRACT The world's forests are experiencing sustained decline, yet how this chronic, non‐stand‐replacing process reshapes bird communities remains uncertain. We synthesized 382 effect sizes from 30 studies (177 species) to quantify abundance responses to forest decline and to test whether heterogeneity is better explained by functional traits, decline attributes (severity, speed, scale), and driver identity (pathogen, insect, abiotic). The pooled mean response was near zero and not distinguishable from zero, but residual heterogeneity was high, indicating strongly structured, group‐specific responses. Nesting guild emerged as the most consistently supported ecological moderator: low‐shrub/ground nesters and tree/canopy‐associated nesters showed the strongest declines, whereas responses among deadwood‐associated guilds were generally less negative and more context‐dependent. In contrast, broad diet and foraging categories had comparatively weak support across the model set, although nectarivores showed negative responses (treated as an exploratory pattern consistent with reduced floral resources). Decline attributes contributed unevenly: severity effects were generally weak, while speed‐related responses differed among drivers and spatial scale showed mixed but sometimes positive associations. Methodological factors (study design and capture method) were also among the strongest predictors of estimated effects, underscoring the importance of observation window and detectability. Overall, forest decline can reorganize avian communities without producing a uniform directional change in abundance, with potential consequences for ecosystem functions where specialized guilds decline.