Temporal variation in shedding of coccidial oocysts: implications for sexual-selection studies

Studies of the effects of parasites on the sexual ornamentation and behavior of animals are becoming increasingly common. A potential difficulty in many studies of this type is the accurate assessment of parasite prevalence within a population and of the parasite burden of individual hosts. We present data that illustrate how a simple factor such as the time of day at which samples are collected can drastically alter the results of parasite assessments. House finches (Carpodacus mexicanus) infected with coccidians of the genus Isospora shed oocysts in their feces predominantly during the late afternoon. The importance of recognizing such circumstances (e.g., diurnal periodicity) is discussed in the light of possible negative effects on studies of host-parasite relationships.