Singing activity and spatial behaviour as sexually selected traits in the nightingale "Luscinia megarhynchos"

I examined singing activity and spatial behaviour of male nightingales with regard to possible functions in mate attraction, territory defence, and sperm competition. Male nightingales sang at night before pair formation. They stopped nocturnal song upon pairing, but resumed it if their mate deserted. In contrast, unmated males sang nocturnal song throughout the breeding season. These findings suggest that nocturnal song plays a role in mate attraction. Diurnal singing activity was highest in the hour before sunrise. This pattern was consistent throughout the breeding cycle and was hardly influenced by mating status, indicating that mate attraction is not the main function of the “dawn chorus” in the nightingale. I tested the alternative hypothesis that territorial males sing at dawn to defend their territory against non-territorial males: I translocated unpaired males to the study site (the „Petite Camargue Alsacienne“) and found that the radio-tagged males made extensive excursions visiting several singing males at dawn, but after dawn remained stationary outside occupied territories. These results suggest that non-territorial males use the dawn chorus to assess singing residents or territory occupancy. It also appears that dawn singing of territorial males is important to announce territory occupancy to non-territorial males and thus to defend the territory. In contrast to the hour before sunrise, singing activity later in morning was strongly influenced by the reproductive state of females and peaked during the egg-laying period. This elevated singing activity may not serve primarily to repel cuckolder males pursuing extrapair copulations, since during the morning hours when egg-laying takes place, copulations are thought to less likely result in fertilization than in the days before egg-laying. Accordingly, male nightingales showed the greatest distances to their mates in the morning hours during the egglaying period. Furthermore, extrapair fertilization may not play a major role in influencing singing and spatial behaviour in the nightingale: the rate of extrapair fertilization was relatively low (7.5% of the young as indicated by microsatellite genotyping), and males showed no distinct mate guarding activity. In contrast, the probability for a male to stay unpaired was, on average, 33% per year. I suggest that the high singing activity as well as frequent extra-territorial excursions of males after the fertile period of females serve to maintain the own territory and to gather information on other territories and territory occupancy. This information may be vital for defending a good territory and attracting a mate in the current and subsequent breeding seasons. As only unpaired males sing regularly at night, the proportion of unpaired males can be assessed by comparing the number of nocturnally and diurnally singing males. In an extended study area of 18 km2, I found that about half of 200 – 240 singing males were unpaired. The male-biased adult sex ratio thus may be a general pattern and play an important role in sexual selection of singing activity and spatial behaviour in the nightingale.