Song as an indicator of parasitism in the sedge warbler.

We studied female choice and reproductive success in a marked population of sedge warblers Acrocephalus schoenobaenus, from 1995 to 1996. Three genera of parasitic blood protozoans, namely Haemoproteus sp. Trypanosoma sp. Plasmodium sp., were identified from blood samples taken from all breeding adults. Relatively high prevalence values of 19.5% in 1995 and 37.5% in 1996 were associated with increased levels of white blood cells relative to the number of red blood cells. Compared with nonparasitized males, parasitized males had significantly lower repertoire sizes in both years of the study; in one year, they also spent less time in song flights and weighed less. They also provisioned their broods at a lower rate. Parasitized females produced the same clutch size as nonparasitized females, although their broods were smaller at 7 days old. We suggest that haematozoan infections may reduce the expression of sexually selected song traits. Furthermore, such infections may influence the standard of parental care provided by males, although further research is needed to determine whether this is mediated through genetic resistance to parasitism or the effects of parasitism upon immediate body condition. Copyright 1999 The Association for the Study of Animal Behaviour.