To evaluate the possibilities and limitations of using white-blood-cell (WBC) counts to characterize investment in immune-system function, we compared the prevalence and intensity of malaria infections and concentrations of WBCs in the American Robin (Turdus migratorius) in Michigan and Missouri and in the Clay- colored Robin (T. grayi) in central Panama. We ascertained infection by polymerase chain reaction (PCR) screening and quantified infection intensity and WBC concentrations by visual inspection of blood smears. Because few parasites were observed on smears, we assumed that most cases of malaria represented chronic, rather than acute, infections. Prevalence of haemosporidian infection (Panama 41%, Missouri 57%, Michigan 63%) did not differ significantly between locations. However, most infections in Panama were undetected on blood smears, whereas more than half were apparent on smears in Michigan and Missouri. Among WBCs, lymphocytes were the most abundant type, followed by heterophils; eosinophils and lymphocytes were more common in the North American sample than in Panama, and their numbers in Michigan were significantly higher in infected than in noninfected individuals. Tropical T. grayi apparently maintained infections at lower intensities but appeared to accomplish this in spite of lower abundances of immune-system cells. Furthermore, analyses of blood smears of Turdus spp. from elsewhere in the tropics revealed a wide range of prevalence, which suggests that either the regional presence of haemosporidians or the ability of hosts to control infections cannot be learned from limited samples. Additional species surveyed in Michigan and Panama revealed no consistent pattern in either infection intensity or WBC concentrations. Infection and response appear to be highly idiosyncratic. In agreement with other authors, we caution that blood parameters are difficult both to interpret and to sam- ple adequately in tests of regional or other effects.

Prevalencia de Malaria y Respuesta de Glóbulos Blancos a la Infección en un Zorzal de la Zona Tropical y Uno de la Zona Templada