Acoustic indices can be driven by habitat characteristics rather than bird diversity

Acoustic indices are widely used to assess biodiversity, including bird diversity. However, numerous studies have reported weak relationships between acoustic indices and biodiversity metrics, with the underlying mechanisms remaining poorly understood. Using point-count survey data and acoustic recordings from a subtropical forest in eastern China, we examined the factors influencing the relationship between acoustic indices and bird diversity, including the point-count radius and single versus multi-index usage. In addition, we used path analyses to quantitatively assess the direct and indirect effects of terrain and vegetation on bird diversity and acoustic indices, with the aim of identifying key factors and mechanisms influencing the relationship between acoustic indices and bird diversity. Our results revealed that a point count survey radius of 75 m yielded the strongest association between acoustic indices and bird diversity, albeit weak (correlation coefficients ranged from -0.45 to 0.48, with many nonsignificant relationships). Moreover, contrary to expectations, combining multiple acoustic indices did not produce a stronger relationship, suggesting that this may be ineffective in measuring bird diversity when individual indices respond in opposing directions across ecological contexts. Path models revealed that compared with bird diversity, habitat characteristics, particularly terrain and canopy height, had a greater influence on acoustic indices. Our findings suggest that acoustic indices may be more strongly influenced by habitat characteristics or other unmeasured taxa, limiting their reliability as measures of bird diversity.