JOURNAL OF AVIAN BIOLOGY 33: 269–275, 2002 The natural plasma testosterone profile of male blue tits during the breeding season and its relation to song output Katharina Foerster, Angelika Poesel, Hansjo ¨rg Kunc and Bart Kempenaers Foerster, K., Poesel, A., Kunc, H. and Kempenaers, B. 2002. The natural plasma testosterone profile of male blue tits during the breeding season and its relation to song output. – J. Avian Biol. 33: 269–275. In male birds, the gonadal hormone testosterone (T) is known to influence territorial and mating behaviour. Plasma levels of T show seasonal fluctuations which vary in relation to mating system and social instability. First, we determined the natural T profile of male blue tits Parus caeruleus during the breeding season. We found that plasma levels of T increased at the onset of nest building. Thus, the increase in circulating T was not associated with territory establishment, nor with the fertile period of the males’ mates. In most individuals, T levels dropped to values close to zero during the period of chick feeding. Second, we investigated the relationship between plasma levels of T and male age, size, and singing behaviour. During the mating period, T levels did not differ between 1 yr old and older males and did not correlate with body size or condition. However, song output during the dawn chorus tended to be positively correlated with T levels. Therefore, if high T levels are costly, song output might be an honest indicator of male quality in blue tits. Finally, we show that plasma levels of T are significantly higher during the night than during the day. This pattern has also been observed in captive non-passerine birds, but its functional significance remains unknown. K. Foerster (correspondence), A. Poesel, H. Kunc and B. Kempenaers, Max Planck Research Centre for Ornithology, Postfach 1564, D-82305 Starnberg (Seewiesen), Germany. E-mail: k.foerster@erl.ornithol.mpg.de Testosterone (T) plays an important role in driving male reproductive behaviour in birds. Temporal pat- terns in the plasma levels of this sex steroid are trig- gered by several factors including photoperiod and food availability (Wingfield 1983), as well as social stimuli (Wingfield et al. 1990). Different T profiles during the breeding season have been related to the social systems and the mating strategies of the observed bird species (Wingfield et al. 1990, Beletsky et al. 1995, Vleck and Brown 1999). Outside the breeding season, when competition for nesting sites and copulation part- ners is low or absent, plasma levels of T are low (nonbreeding baseline, sensu Wingfield et al. 1990). When breeding territories are established and when mating takes place, T levels rise up to 20-fold to a breeding baseline (e.g. Ball and Wingfield 1987, Vleck and Brown 1999, Wada et al. 1999). Within the breed- ing season, large fluctuations in T levels are observed, for instance during social challenges (Wingfield et al. 1990). In all temperate passerines with paternal care, T levels drop rapidly during the incubation phase and are low during the period of nestling feeding (e.g. Ball and Wingfield 1987, Vleck and Brown 1999). T levels may show repeated peaks or remain elevated over the entire breeding season, when multiple clutches, extra-pair cop- ulations, or polygyny are frequent (e.g. Hegner and Wingfield 1986, Beletsky et al. 1995, Wada et al. 1999). As a consequence of the T-mediated trade-off between sexual behaviour and parental care (Hegner and Wingfield 1987, Beletsky et al. 1995, Raouf et al. 1997), high T levels during the late breeding season enable males of these species to acquire additional breeding or copulation partners, at the cost of reduced paternal behaviour. T is known to affect also the development of mor- phological, sexually selected traits (e.g. Zuk et al. 1995, Eens et al. 2000, Evans et al. 2000, Peters et al. 2000). Similarly, the role of T in the expression of behavioural © JOURNAL OF AVIAN BIOLOGY JOURNAL OF AVIAN BIOLOGY 33:3 (2002) 269