Nocturnal playback experiments: The response of two European species of birds to singing of foreign male at night.

Recent studies have shown that nocturnal singing in diurnal birds is a common phenomenon, however, the understanding of the mechanisms, functions and consequences of this behaviour has been lacking. We focused on the night singing of two diurnal songbirds-the yellowhammer and the common chaffinch that are widely distributed in Europe. We conducted day and night playback experiments, during which we broadcast songs of an unfamiliar male to the territory holder at two different stages of the breeding season, to examine whether the night singing in species which normally do not sing at night elicits responses from their conspecifics. We hypothesized that if nocturnal singing had no function, birds would ignore the night-time playback and respond only to the daytime intrusion. Otherwise, a response to the night-time playback would suggest that night singing may increase reproductive success but is limited by ecological factors. We found that, in contrast to the diurnal experiment, neither species responded vocally to the nocturnal playback. In yellowhammers, the probability of flights was higher during and after the playback than before it. This pattern was similar both during the day and night and did not differ between the stages of the season. For the common chaffinches, the probability of flight was low at night and constant across treatments, in contrast to the day when we observed more flights during the playback stage than before and after it. The playback of either species' songs at night caused the approach of predators, which was not observed during the day. The observed discrepancy in the probability of flights between the species suggests that nocturnal singing is a functional trait that affects other individuals in a specific context. Nocturnal singing does not seem to be a simple behaviour that extends during the day; it is a complex mechanism potentially dependent on specific conditions related to intraspecific communication, predatory pressure, local female access, or species' evolutionary history. We emphasise that those ecological and evolutionary factors need to be taken into account to understand this phenomenon widely.