Today's parasite is Isospora plectrophenaxia. A few weeks ago, you met a related species - Isospora lesouefi - the coccidian parasite found in the Regent Honeyeater which keeps a daily timetable, shedding most of its oocysts (the parasite's infective stage) in the afternoon. This is a well-described phenomenon among different species of Isospora - the parasite's shedding schedule appears to be calibrated by the light-dark cycle experienced by the bird host throughout the day. Indeed, experiments conducted on Isospora in house sparrow shows that if you disrupt the circadian rhythm of the host, you also mess up the parasite's shedding schedule.
Under natural condition, the usual light-dark cycle works just fine for most species of Isospora. But I. plectrophenaxia is found in the Snow Bunting (Plectrophenax nivalis) - a bird living in the High Arctic where there is perpetual sunlight during summer. So you'd think the shedding schedule of I. plectrophenaxia would be all messed up, right? Not so, researchers found that the parasite continues to stick to its regular regime of late afternoon shedding, just like all the other Isospora. At the moment researchers are unsure how I. plectrophenaxia is able to perform this feat. Perhaps this species is more sensitive to very low concentration of melatonin - the chemical secreted by the pineal organ which coordinates the bird's circadian rhythm, or perhaps it sets its timetable on different level of UV (ultraviolet) radiation exposure, which still varies throughout the Arctic summer day. Hopefully, ongoing research on this host-parasite system will shed further light on this little mystery, so watch this space!
Dolnik O.V., Metzger B.J., Loonen M.J. (2011) Keeping the clock set under the midnight sun: diurnal periodicity and synchrony of avian Isospora parasites cycle in the High Arctic. Parasitology 138:1077-1081.