Phenology underlies inconsistent urbanization effects on avian malaria ( <i>Leucocytozoon</i> sp.) in juvenile songbirds

Abstract Urbanization can modify species interactions, including those between parasites and their hosts. In birds, urbanization can either increase or decrease avian malaria (haemosporidian) infection, depending on host species, parasite, or study location. However, temporal coordination between parasites and hosts, which may impact infection outcomes, has not been studied in urban ecology. To fill this gap, we collected blood samples from wild blue tit nestlings ( Cyanistes caeruleus ) in urban and forest habitats to examine how their hatch dates affected the prevalence and intensity of haemosporidian infection. To separate parasites and quantify parasite load, we newly developed a species‐specific quantitative polymerase chain reaction (qPCR) assay. We found that Leucocytozoon prevalence was driven by nestling hatch date: nestlings that hatched earlier in the spring had a lower probability of being infected, independent of habitat type. In the few heavily infected nestlings, intensity of infection was associated with low body mass, suggesting fitness risks of infection. These results highlight the importance of breeding early to avoid early‐life infection with malaria parasites. Underappreciated phenological effects may underpin other ecological impacts of urbanization.