Young male blackcaps with blood parasite coinfections cope with oxidative stress favouring anthocyanin‐rich food during migratory fattening

Parasites may alter host physiology, which may promote behavioural adaptations to counteract their effect. Adaptive feeding may help individuals to cope with infection, especially during physiologically demanding life stages. For instance, migrating birds need fuel for long‐distance flights and repair oxidative damage caused by intense aerobic exercise, and parasites may influence on how individuals balance these needs. Infected birds may face increased oxidative challenges, which could induce them to favour antioxidant defences over other needs, such as fattening. We tested whether migrating birds can adaptively choose food according to their needs, favouring dietary antioxidants to cope with oxidative stress caused by haemosporidian blood parasites during migration. During autumn migration, we mist‐netted young male Eurasian blackcaps Sylvia atricapilla stopping over in central Spain. We placed the birds in cages where they were offered fat and anthocyanin‐enriched food alternatives. We measured preference for each food offer. We tested their infections with haemosporidian parasites by PCR techniques and their parasitaemia by blood smear inspection. We also measured physiological variables that account for nutritional and oxidative status in red blood cells and plasma. We found that birds with multiple infections favoured anthocyanin‐enriched food controlling for an effect of body mass on food preference (lean blackcaps preferred anthocyanins, likely because they are urged to repair oxidative damage upon arrival on stopover with depleted energy reserves). Haemosporidian‐infected birds had a lower antioxidant capacity of plasma, although no effect of infections on oxidative damage was detected, and individuals with more oxidative damage preferred anthocyanin‐enriched food. Our results suggest that haemosporidian infections may increase individual antioxidant needs, which could affect migratory performance if the urge to find dietary antioxidants reduces the rate of fuel consumption.