Singing behaviour of Ruby-crowned Kinglets (Regulus calendula) in relation to time-of-day, time-of-year, and social context

Observational field studies provide insight on the multifunctional nature of birdsong. For example, if song production were limited to pre-fertilization, then that would suggest a mate attraction function. If it were used throughout the breeding season and in response to intruding males, then that would suggest a territorial defence function. In the present study, we determined the daily and seasonal singing patterns of male Ruby-crowned Kinglets (Regulus calendula) in Labrador, Canada, using microphone arrays in two breeding seasons. Using a playback experiment, we simulated a territorial intrusion to compare the structure of songs produced while defending a territory to the structure of songs produced during solo and contest singing. Singing peaked in the early part of the breeding season and then declined continuously for the remainder of the season, which suggests that the songs function in mate attraction. Singing peaked 2-3 h after dawn, and then declined steadily until it stopped at 2200 h. Some nocturnal singing was observed, but no dawn singing was observed. A high probability of signal overlap by heterospecific songs at dawn would hinder signal recognition and explain the observed delay in peak singing activity. Vocal responses to playback suggested a function in territory defence. However, there were no significant differences in the duty cycle, frequency modulation, and bandwidth of songs in relation to the context of song production, though songs were shorter in the intrusion context than during solo singing. Overall, the study provides the first quantitative description of the effects of time of day, time of year, and social context on singing behaviour in this understudied species.