Circadian variation in shedding of the oocysts of Isospora turdi (Apicomplexa) in blackbirds (Turdusmerula): An adaptative trait against desiccation and ultraviolet radiation

Many parasite species spend part of their life cycle in the external environment waiting for a new host. Emergence of parasites often occurs once a day, which may help to minimise mortality in an inhospitable environment and increase transition rates. Many intestinal parasites in birds are released in faeces only in the late afternoon. However, the adaptative significance of this pattern is unclear. One hypothesis is that a particular time of emergence may prevent parasite desiccation and therefore increase the parasite’s life expectancy in the external environment. We tested this hypothesis experimentally using the blackbird (Turdus merula) infected with Isospora turdi (Protozoa: Apicomplexa). We found that short exposure of faeces to natural sunlight has a dramatic effect on oocyst survival. This appears to be due to the effect of warmth and ultraviolet (UV) radiation with UVB waves being more damaging than UVA. Oocysts contained in faeces shed in water are protected from the effect of sunlight. Together, these results suggest that the release of oocysts in the late afternoon is an adaptative trait to avoid desiccation and UV radiation, thus reducing mortality of the oocysts in the external environment.