Data from: A rare study from the wintering grounds provides insight into the costs of malaria infection for migratory birds

Malaria parasites can have strong effects on the population dynamics and
evolution of migratory bird species. In many species, parasite
transmission occurs on the wintering grounds, but studies to determine the
consequences of infection have taken place during the breeding season,
when malaria parasites circulate at chronic levels. We examined the
predictors of malarial infections for great reed warblers during the
northern winter in Africa, where active parasite transmission is thought
to occur and naïve individuals experience acute infections. Counter to
expectations, we found that winter infection intensities were lower than
those encountered on the breeding grounds. One potential explanation is
that reduced immune function during breeding allows parasites to persist
at higher chronic intensities. We found no relationships between the
incidence or intensity of infection on condition (as measured by scaled
mass index, plasma metabolites, and feather corticosterone), spring
migration departure dates, or home range sizes. We also tested a
prediction of the Hamilton–Zuk hypothesis and found that male ornament
(song) quality was unrelated to parasitic infection status. Overall, our
results provide the first evidence that long-distance migrants captured on
their wintering grounds are in the chronic stage of infection, and suggest
that winter studies may fare no better than breeding studies at
determining the costs of acute malarial infection for great reed warblers.