Eurasian Blackbirds (<i>Turdus merula</i>) and their gastrointestinal parasites

Abstract

Parasites are increasingly being recognized as potent agents of selection. Gastrointestinal parasites, however, have largely been ignored. The aim of the present thesis was to contribute to the still scarce knowledge about gastrointestinal parasites in ecological, evolutionary and behavioural studies and to critically elucidate methodological aspects of indirect parasitic assays.<br /> Fecal samples were collected from an urban, individually marked population of Eurasian Blackbirds (Turdus merula L.,1758) and were examined for eggs and oocysts of gastrointestinal parasites. Four parasitic taxa commonly occurred in the population: protozoans of the genus Isospora, nematodes of the genus Capillaria, cestodes and acanthocephalans. Isospora spp. and Capillaria spp. were the most frequent parasites, infecting 53 % and 56 % of birds, respectively. These results markedly differed from a similar study conducted 35 years ago (Binder 1971) and explicitly demonstrate, that the parasite communities and prevalences assessed from single populations can not be taken as representative for the whole species, as has repeatedly been done in the past.<br /> It was investigated whether these parasites impose fitness costs on their hosts. It is a central assumption of life-history theory that the availability of energy or resources is limited. Thus, the investment in one costly trait can only occur at the expense of another costly trait (Stearns 1992). In adult male blackbirds I found a cost of parasitism. Males that had invested more heavily in the feeding of nestlings, were also more likely to be infected with gastrointestinal parasites. Female infection prevalence was very high (95%) and significantly higher than in males. Since females are alone responsible for nest building, egg laying and incubating they probably incur high reproductive costs already prior to the nestling feeding stage. In nestlings no direct cost of parasitism on aspects of somatic growth could be found. Quite on the contrary, nestling size, measured as tarsus length and wing length were positively associated with prevalence of coccidian infection. I conclude that infections with Isospora spp. do not impose the predicted trade-off on blackbird nestlings. Rather, infections with Isopora spp. seem to be a function of feeding rate. Thus, an alternative trade-off, namely between energy intake and risk of infection seems to become of primary importance for blackbird nestlings.<br /> It is a still commonly practised method to assess a bird’s infection status by the examination of a single sample, although it is known that the failure to find parasitic stages does not reliably indicate the absence of an infection. Several properties of the host and of the parasites potentially influence the probability to detect parasitic stages in samples. One such property is the temporal fluctuation of oocyst or egg shedding by parasites. In fecal samples of adult and nestling blackbirds I found oocysts of Isospora spp. to be shedded more frequently and at higher intensities in the afternoon. This periodic phenomenon, the diurnal shedding of isosporan oocysts in the feces, is in congruence with results on Isospora oocyst shedding in other bird species. In order to give future investigators a tool to estimate the quality of their indirect method, I developed a model in which the calculation of the parameter pinf determines the probabilty to falsely denote an infected individual as “not infected” if no parasites can be found in n samples collected from this individual. The model showed that in the blackbird population it was not possible to reliably (with a probability of 95%) denote an individual as not infected after the evaluation of a single fecal sample. These results suggest that great consciousness and caution have to be used in the methodological planning of studies involving indirect parasitic assays.