Résumé
ABSTRACT Parasitic infections are ubiquitous in nature, but their consequences are often difficult to evaluate in wildlife. This is particularly the case for chronic infections for which fitness impacts can be subtle or confounded with other intrinsic and extrinsic factors. Infections by Babesia (Apicomplexa, Piroplasmidae) are widespread in wildlife, but little is known about their impacts on host health. Here, we evaluated the dynamics and possible fitness consequences of infection by Babesia sp. YLG, a recently described parasite infecting yellow‐legged gulls ( Larus michahellis ). Using a qPCR assay developed for Babesia sp. YLG, we found that prevalence was much higher in chicks compared with adults. In chicks, infection depended on the colony and was tightly related to nest infestation by the tick vector Ornithodoros ( Alectorobius ) maritimus ; this was not the case for adult birds. Infection intensities changed over time in chicks, increasing up to approximately 37 days old and then declining. Infection was associated with reduced body condition and fledging success for adults. In chicks, infection was significantly linked to reduced growth rates, even after accounting for the additive effect of ticks. However, no relationship was detected between infection and fledging date, migration strategy or flight distances measured on a subset of GPS‐tagged juvenile birds. Through its subtle effects on fitness and reproductive success, infection by Babesia sp. YLG may have significant consequences for host demography and work focused on long‐term dynamics is now called for to better understand how this parasite may shape the ecology and evolution of seabird populations.