Prevalence and intensity of hematozoan infections in a population of American kestrels

Interest in hematozoan parasites has been considerable in recent years, mostly as a result of Hamilton and Zuk's idea that parasites influence the expression of sexually selected traits. However, little is known about the basic patterns of parasitism and the dynamics of host-parasite relationships. We describe the patterns of blood parasitism in American kestrels (Falco sparverius) sampled throughout the breeding seasons of 1994 and 1995, and investigate the influence of several variables on parasite load. Parasite prevalences in kestrels were high, ranging from 75 to 94% depending on the sex of the birds and stage of the breeding season. Prevalence increased with date, indicating either active parasite transmission or relapses of chronic infections. Parasite intensity also increased with date, but these effects were sex- and year-specific. We detected no sex differences in either prevalence or intensity, but prevalences were higher in young (second calendar year) birds than in older birds. Because this effect was evident early in the breeding season, we suggest that it was due to differential recrudescence of chronic infections between age-classes. Food supply had no effect on parasite load. Among birds that were sampled twice in the same year, parasite status remained relatively constant, although some birds acquired infections while others lost them. Change in intensity between samples was dependent on year; nonetheless, intensities remained relatively stable throughout the breeding season. Kestrels sampled in both years had consistent parasite loads. Parasites w,ere detected in only 3% of kestrel nestlings.