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.