A tropical bird can use the equatorial change in sunrise and sunset times to synchronize its circannual clock

At higher latitudes, most organisms use the periodic changes in day length to time their annual life cycle. At the equator, changes in day length are minimal, and it is unknown which cues organisms use to synchronize their underlying circannual rhythms to environmental conditions. Here, we demonstrate that the African stonechat (Saxicola torquatus axillaris)-an equatorial songbird-can use subtle solar cues for the annual timing of postnuptial moult, a reliable marker of the circannual cycle. We compared four groups that were kept over more than 3 years: (i) a control group maintained under constant equatorial day length, (ii) a 12-month solar time group maintained under equatorial day length, but including a simulation of the annual periodic change in sunrise and sunset times (solar time), (iii) a 14-month solar time group similar to the previous group but with an extended solar time cycle and (iv) a group maintained under a European temperate photoperiod. Within all 3 years, 12-month solar time birds were significantly more synchronized than controls and 14-month solar time birds. Furthermore, the moult of 12-month solar time birds occurred during the same time of the year as that of free-living Kenyan conspecifics. Thus, our data indicate that stonechats may use the subtle periodic pattern of sunrise and sunset at the equator to synchronize their circannual clock.