Seasonal fluctuations in plasma testosterone levels and diurnal song activity in free-living male great tits.

The annual course of plasma testosterone (T) and song activity during the day was examined in free-living male great tits (Parus major) in a single year and in a single population. We provide the first study in this species investigating plasma T during the breeding season among and within breeding stages. As expected for a temperate-zone monogamous species, plasma T levels showed a pronounced peak in March at the start of the breeding season and fast decreasing levels thereafter to moderate levels in the parental phase. Peak plasma T levels coincided with the period of intense territoriality but were not sustained during the egg laying period, suggesting that the annual T peak is more related to territoriality than to mate fertility or dawn song. After basal plasma T levels during summer and autumn, T levels started to increase from December onwards, possibly related to the reappearance of territoriality and pair formation in early winter. Within the feeding stage, males breeding later in the season had lower T levels than males breeding earlier, independent of nestling age, male condition, or the percentage of fertile females in the population. Also, during the nestling stage, T levels increased as nestlings were older, while controlling for time in season, male body condition, and availability of fertile females. The annual T pattern was highly correlated with the annual pattern of day-time song activity in the population. However, a small resurgence in autumn of day-time song activity in the population was not accompanied by a noticeable increase in plasma T in the captured males. Peak plasma T levels did not occur during the egg laying stage when dawn song activity is maximal, suggesting that there is no strong causal relationship between the two.