Soft song during aggressive interactions: Seasonal changes and endocrine correlates in song sparrows

► We examined soft song and endocrine correlates in breeding and non-breeding birds. ► Soft song increased in the non-breeding season relative to broadcast song. ► Increased soft song correlated negatively with plasma testosterone concentrations. ► Soft song trill minimum frequency and buzz repetition rate differed between seasons. ► Soft song in wintering birds may reduce predation and energetic costs of singing.