Effects of hematozoan parasites on condition and return rates of American Kestrels

We evaluated the relationship between brood parasites and body condition of American Kestrels (Falco sparverius) during the breeding season. Females t hat were infected with at feast one species of parasite were in poorer cond ition than those without parasites during incubation but not prior to egg l aying. We suggest that the relationship between parasitism and condition wa s masked before laying because of large increases in body mass of females d uring egg formation. Reduced condition of males during incubation also was associated with higher intensity of infections by Haemoproteus in one of tw o years. The negative association between condition and intensity of infect ion suggests that blood parasites impose costs on kestrels owing to competi tion for nutrients or allocation of energy by hosts to immune function or t issue repair. Alternatively, kestrels in poor condition may be more likely to have relapses of chronic infections, or they may be less able to control new infections because of resource limitations. In contrast to results dur ing incubation, during the prelaying period the prevalence of parasites ten ded to be higher, and in one year infections were more intense, among males in good condition. One possible explanation for these results is that body condition of males during courtship is an important determinant of the qua lity of mate they are able to obtain, and males may be accumulating body re serves at the expense of decreased immune function. Return rates of female kestrels to the study area declined as the intensity of their Haemoproteus infections increased, suggesting that blood parasitism is associated with r educed survival or increased dispersal probability.