The silver-beaked tanager ( Ramphocelus carbo), family Thraupidae, is a non-migratory, sexually dimorphic species 21, 22, 23. 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. On the other hand, the variation in twilight time is approximately 32 minutes over the year. In this region, the variation in day length (from sunrise to sunset) over the whole year is approximately 9 minutes. We tracked the daily and seasonal timing of dawn-song continuously for 19 months in free-living males of the eastern Amazon of Brazil. Dawn-song is produced by male silver-beaked tanagers as a discrete daily performance at twilight, and this behavior is expressed only during the breeding season 20. Here, we studied whether small changes in daylight are used to time dawn-song in an equatorial songbird, the silver-beaked tanager ( Ramphocelus carbo). However, 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. In temperate-zone songbirds, regular variations of daylight are essential for timing and activating song behavior, on a daily scale as well as seasonally 14, 15, 16, 17, 18, 19. Song behavior is primarily expressed during the breeding stage 12 and in some species peak song output is highest before the rising of the sun, a phenomenon called the ‘dawn-song’ 13. The song of songbirds, a group which comprise about half of all bird species in the oscine suborder of the passerines, functions in sexual signaling and territorial defense. Laboratory experiments suggest that tropical birds are able to use changes of day length at lower latitudes 10, or use changes in solar time of about 30 minutes to entrain endogenous rhythms near the equator 11. On the other hand, photoperiod near the equator remains almost constant across the year, and the question of which cues regulate behavioural rhythms in equatorial organisms remains less explored. 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. Accordingly, many animals use day length changes as a reliable cue to synchronize their behaviors with the environment. Life in the biosphere has evolved under the influence of cyclical changes in solar daylight caused by the Earth’s rotations and revolutions.

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