Parental Effort and Blood Parasitism in Tengmalm's Owl: Effects of Natural and Experimental Variation in Food Abundance

Abstract

We investigated the association between parental effort and susceptibility to haematozoan parasites in vole-eating Tengmalm's owls (Aegolius funereus) from 1993 to 1995. In a poor vole year (1993), almost all breeding Tengmalm's owls were infected with Trypanosoma avium, whereas in a good vole year (1994), only a few owls were infected. In a moderate vole year (1995), we found an intermediate prevalence of trypanosomes. In the moderate vole year, trypanosome-infected females were in poorer condition than were uninfected females. In the same moderate vole year, high parental effort was associated with increased susceptibility to haematozoan parasites for both genders, whereas in a good vole year no such association was found. In two breeding seasons (1996-1997) of relatively low vole abundance we tested whether supplementary food decreased parasite loads. In accordance with correlative data, trypanosome prevalence was lower among supplemented than control females. Our results support a hypothesis of a trade-off between parental effort and immunocompetence, and emphasize the importance of varying environmental conditions and physical condition of individuals on susceptibility to haematozoan infections.