In most songbird species of the temperate zones seasonal events such as breeding are associated with seasonal fluctuations in steroid hormone levels and reproductive behaviour such as singing. In a long-term study on an aviary population of domesticated canaries, one of the most widely used models to study the neuroendocrine basis of behavioural plasticity, we investigated this hormone-behaviour relationship both within and between seasons. We found that song parameters such as song length, the proportion of non-repeated syllable types and sexually attractive syllable types changed seasonally together with circulating plasma levels of testosterone (T), and are therefore likely to be hormone dependent. In contrast, repertoire size remained constant throughout seasons. Within the breeding season, however, song behaviour was maintained constant despite fluctuations in plasma T levels suggesting a threshold effect for certain song features. These results show that this complex hormone dependent regulation of song behaviour can only be fully understood by investigating different physiological and seasonal stages.