Distress calls reflect poxvirus infection in lesser short-toed lark Calandrella rufescens

Several studies have highlighted the association between bird song and parasite load, but there is no evidence regarding the relationships among pathogens and alarm or distress calls, which are used in antipredator strategies. We analyzed the association between virus infection and the distress calls of lesser short-toed lark (Calandrella rufescens), addressing the relationships between call acoustic properties, presence of poxvirus lesions, and other measurements of bird health (body condition, T-cell–mediated immune response, heterophils-to-lymphocytes ratio [H/L ratio], and blood parasites). The study was carried out in Fuerteventura (Canary Islands), where 55% of lesser short-toed larks were infected. Pox infection was associated with changes in the spectrotemporal structure of lark distress calls and affected the condition-dependent nature of these signals. Virus bearers uttered significantly shorter and lower pitched calls than virus-free birds, giving even shorter calls when subject to greater physiological stress (higher H/L ratio), whereas virus-free individuals did the opposite. Call harshness was positively correlated with bird immune and body condition, independently of virus infection, whereas call pulse rate decreased in stressed birds. We hypothesize that healthy birds might reveal their ability to bear the costs of antipredator defense by means of long, harsh, and fast modulated distress calls. Infection not only affects individual state and morphology by means of gross lesions but also may alter a signal used in an antipredator context, thus potentially reducing bird fitness both directly (through disease) and indirectly (through increased predation).