Blood parasites and male fitness in the pied flycatcher.

In vertebrates the effect of parasites on host ecology has almost been ignored. Recently the view that well-adapted parasites do not harm their hosts has been challenged and there is growing evidence that parasites do have a present-day effect on a great variety of host fitness components. The pied flycatcher is a small migratory passcrine bird. Any decrease in condition caused by disease should affect its ability to cope with physical demands of migration. Here we examine whether blood parasites have any effect on male arrival time. Males infected with Trypanosoma arrived on average 2 days later than males with no Trypanosoma infection. Infected males also had shorted tails and tended to have shorter wings. By contrast, there was no difference in male arrival time between males infected with Haemoproteus and healthy males. It seems that Trypanosoma infection lowered male condition and consequently the ability to moult and migrate. The difference in length of feathers may have generated the difference in arrival times. Early arrival is highly important for males, since only the first males become polygynous and breeding prospects deteriorate rapidly with any delay in egg laying. Estimated reduction in breeding success for infected males was about 20%.