As with many other songbirds, the singing behaviour of adult nightingales Luscinia megarhynchos is distinguished by a temporally very regular alternation of songs (strophes) and silent inter-song intervals (pauses). These pauses are essential for the vocal interaction among conspecifics, as they allow them to listen to and respond .to the songs of other birds (Hultsch & Todt, 1982, Beh. Ecol. Sociobiol., 11). In the past, many studies have dealt with how song patterns are acquired and developed. In contrast, the ontogenetic development of the time structure of singing has been a neglected issue. In nightingales, the most conspicuous change during this process occurs towards the end of the first year of birds, when they gradually switch from juvenile continuous singing to the adult discontinuous singing style. Our study examines the characteristics of this transition and the rules underlying its temporal patterning! We investigated the singing of seven male birds who had been trained as fledglings with different song strings (duration of intervals between successive master-song types: 4s). Six of these birds were kept in auditory isolation after the tutoring period. To check for influences of social experience on song development, one male was housed in auditory contact with other nightingales. A further male had no exposure to any song (acoustic isolate). In all tutored birds (i.e. including the 'contact' male), the transition from continuous to discontinuous singing started at week 45 posthatching (SD: 3 days). It lasted between 18 and 37 days (early April to early May). In the acoustic isolate, the beginning of temporal crystallisation was delayed (week 47 p.h.), but it took him only 13 days to develop the adult singing style. To further characterise the transition process, we measured for each male the duration of app. 100 consecutive inter-song intervals (0.85s-12s), starting with the first singing bout in the morning of 10 days during the period mentioned above. This analysis revealed that the transition process can be divided into two distinct phases. The first phase is characterised by a gradual increase of the pause durations from 1.7s to 2.8s (medians). This increase succeeded within max. 7 days. Interestingly it was followed by a marked decrease in pause durations, during which 5 birds even regressed to continuous singing again. In the second phase, the duration of inter-song pauses increased again and progressed to the adult time structure within 20.3 days (SD: 4.8 days). Inter-song pause duration ranged from 2.4s to 4.5s (medians) then. Temporal development in both the 'contact' male and the isolate male was consistent with these characteristics, i.e. they did not differ significantly in any aspect from the other birds. We conclude from these findings that the ontogeny of the adult time structure of singing follows a developmental program in which intrinsic, age related variables play an important role. In particular, the evidence from the onset and course of the transition from continuous to discontinuous singing, which was very consistent among the tutored birds, supports the assumption of an age trajectory in development. This trajectory, however, may not be independent from feedback from the individual's performance skill, as suggested by the delay in transition onset in the acoustic isolate. Further experiments are needed to substantiate this finding. Also, the ontogenetic regression in pause durations (end of phase 1) has to be examined for covariation with other variables in the singing behaviour during that period. Concomitant with temporal regression could be, for instance, a decrease in singing activity (Kopp, 1992, Ber., 125. Jahrestagung DO-G), or structural 'quality' of the performance. Elucidating the interaction or independence of such variables will further our understanding of processes and mechanisms involved in behavioural development.
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