Abstract
Behaving in accordance with natural cycles is essential for survival. Birds in the temperate regions use the changes of day length to time their behavior. However, at equatorial latitudes the photoperiod remains almost constant throughout the year, and it is unclear which cues songbirds use to regulate behaviors, such as singing. Here, we investigated the timing of dawn-song of male silver-beaked tanagers in the equatorial lowland Amazonas over two years. In this region, birds experience around nine minutes of annual day length variation, with sunrise times varying by 32 minutes over the year. We show that the seasonal timing of dawn-song was highly regular between years, and was strongly correlated with slight increases in day length. During the singing season the daily dawn-song onset was precisely aligned to variations in twilight time. Thus, although photoperiodic changes near the equator are minimal, songbirds can use day length variation to time singing.
Highlights
Life in the biosphere has evolved under the influence of cyclical changes in solar daylight caused by the Earth’s rotations and revolutions
Our results show that daily dawn-song onset in males was precisely aligned to regular fluctuations in light intensity determined by the twilight time, and the seasonal timing of song was highly regular between years and closely correlated to slight increases in day-length
We tested whether the probability of dawn-song to occur throughout the years was dependent on the monthly rainfall rate, the civil twilight time, and the day length as fixed effects
Summary
Life in the biosphere has evolved under the influence of cyclical changes in solar daylight caused by the Earth’s rotations and revolutions. Birds have historically been reference models for understanding animal responses to changes in daylight, including the relation of breeding biology to variations in light intensity and day length[1,2,3,4,5,6,7,8], but research has greatly focused on species inhabiting temperate regions[9]. Despite the fact that the great majority of bird species live in lower latitudes, the significance of daylight changes for seasonal timing of song in equatorial songbirds has scarcely been studied in longitudinal field investigations. We tracked the daily and seasonal timing of dawn-song continuously for 19 months in free-living males of the eastern Amazon of Brazil In this region, the variation in day length (from sunrise to sunset) over the whole year is approximately 9 minutes. Our results show that daily dawn-song onset in males was precisely aligned to regular fluctuations in light intensity determined by the twilight time, and the seasonal timing of song was highly regular between years and closely correlated to slight increases in day-length
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