Blood parasites and nest defense behaviour of Tengmalm's owls.

Infectious diseases are expected to negatively influence essential life history traits of an individual, because investment in immunological response occurs at the expense of reduced investment in other functions. Here we present the first observational evidence that the prevalence of blood parasites is negatively associated with avian nest defense. Because the defense of offspring entails a risk of serious physical harm to the parent, it is also assumed to be a good estimate of parental investment. In both 1994 and 1995, the nest defense intensity of male Tengmalm's owls (Aegolius funereus) against a live American mink (Mustela vison) was strongly curtailed in parents infected by Trypanosoma avium blood parasites. Our data suggests that investment in reproduction can be negatively affected by parasitaemia, and that host-parasite interactions may potentially modify hosts' life-history traits, making it important to consider the costs of parasitism in future studies.