The relative stability of chronic Isospora sylvianthina (Protozoa: Apicomplexa) infection in blackcaps (Sylvia atricapilla): evaluation of a simplified method of estimating isosporan infection intensity in passerine birds.

Estimation of intensity of coccidia infection in wild birds is essential for the studies of the parasite impact on birds' natural populations. In this paper I propose a method for quantification of coccidia oocyst output from passerine birds applicable in field investigations because it is based on one faeces droplet per bird only. This method was checked under controlled laboratory conditions on blackcaps, chronically infected with Isospora sylvianthina, and was proved to be sensitive enough for the detection of between-individual differences in parasite load. I also show that individual variation in the intensity of isosporan infection in captive blackcaps is relatively stable in time, both in the scale of a few consecutive hours and in that of consecutive days. Hence, the study shows that, in passerine birds, single measurements of oocyst production, when collected in the proper time of day, can be used to characterise some more permanent aspects of an individual's coccidia infection status.