Other questions hang over the role of external factors in regulating the annual cycles of birds that live year-round near the equator and experience more or less constant daylengths throughout their lives. Some such species live in relatively aseasonal habitats, such as rainforest, whereas others live in open country, where seasonal rainfall causes seasonal fluctuations in food supplies. In these latter species, the regular wet-dry seasons and associated food changes may act as external time-keepers. Nevertheless, some equatorial and desert birds show endogenous circannual rhythms when kept in constant conditions in captivity (Gwinner & Helm 2003), and can also respond to photoperiodic changes (Lofts 1964, Gwinner & Scheuerlein 1999, Bentley et al. 2000). In Panama at 9°N, the Spotted Antbird Hylophylax naevoides experiences daylengths that vary only between a minimum of 12 hours in December and a maximum of 13 hours in June. Yet in experimental conditions, individuals responded by song to an increase in photoperiod of only 17 minutes, and by gonad growth to an increase of 28 minutes (Hau et al. 1998).
Daytime light intensity, rather than duration, could act as a time-keeper for some equatorial birds. It changes greatly with cloud cover, and hence reflects the cycle of dry and wet seasons in relation to which many tropical birds breed. The experimental exposure of African Stonechats Saxicola torquata to a constant 12.25-hour photoperiod, but with cyclic changes in daytime light intensity, caused their gonadal and moult rhythms to become synchronised with the light intensity cycle. Control birds exposed to the same photoperiod, but to a constant high light intensity, were not synchronised, and showed variable responses. These results suggest a role for daytime light intensity as a circannual timing mechanism, and also provide a possible explanation for the strong responsiveness of African
Stonechats to photoperiodic change: light intensity and daylength may act syner-gistically on one and the same mechanism (Gwinner & Helm 2003).
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