Patterns of song plasticity in passerine birds beyond the first year are poorly studied. In general, songbirds are divided into two categories: open-ended learners and closed-ended learners, depending on the pattern of age-related vocal plasticity. However, recent work based on longitudinal studies revealed a broader range of flexibility of song changes in adulthood. Serins sing very complex songs with large repertoires which are delivered in a very rigid way with little structural modification. However, there is little information on how serin song changes with age. We studied vocal plasticity in wild adult serins by recording male song over 2 years. The analyses show that male songs have only limited variation between years, with no increase in repertoire size and relatively small changes in their structural characteristics. Syllable production was very consistent within and between years with very little structural variation. New syllables represented only 8% of the repertoire, and they appeared to emerge from fusion or splitting of pre-existing syllables. We conclude that serin song while structurally complex has a very limited age-related plasticity after the first year. We hypothesise that this structural stability is a consequence of selection for performance consistency.