Season-dependent trait–environment filtering influences interaction networks in mixed-species bird flocks

Seasonal variation can strongly influence species interactions, yet its effects on interaction networks remain poorly understood in tropical and subtropical systems. We surveyed mixed-species bird flocks (MSFs) across 20 2 × 2 km grid cells in a subtropical forest in southern China during breeding and non-breeding seasons, quantified seasonal changes in flock composition and network structure, and examined how species traits and environmental factors drive these changes. More species and individuals joined mixed-species bird flocks in the non-breeding season, producing more connected and cohesive networks. During the breeding season, network structure was associated with breeding traits: species with larger clutch sizes showed higher association strength, but overall network connectivity and cohesion declined, suggesting trade-offs imposed by parental care. Network structure in the non-breeding season was related to morphological traits, with smaller species forming more cohesive networks. Environmental effects were also season dependent. Connectivity increased with elevation, forest cover and canopy cover in the breeding season, whereas in the non-breeding season networks were most connected and cohesive at low elevations with high tree species richness and vegetation productivity. Our results demonstrate the importance of seasonal effects on mixed-species bird flocks and highlight the need to maintain mountain connectivity, as communities may rely on different parts of mountains across seasons.