Blackcaps (Sylvia atricapilla) with blood parasites have impaired oxidative status during autumn migration

Parasites decrease host fitness, more so during physiologically challenging life stages such as migration. However, parasites are unevenly distributed among individuals, and hosts with different physiological condition may face different costs of migration with infections. We tested whether the relationship between infection by haemosporidian parasites and oxidative balance during autumn migration in blackcaps depends on individual age and sex, two major sources of variation in prevalence and physiological condition. We captured blackcaps at stopover in central Spain and measured infection status and physiological parameters. Blackcaps physiologically more prepared to depart from stopover had lower antioxidant capacity of plasma, probably because of physiological adjustments prior to the upcoming flight stage. Plasmodium parasites occurred only in coinfections, which were more frequent among young blackcaps. Regardless of individual sex and age, coinfected birds had lower antioxidant capacity of plasma, and blackcaps coinfected with Plasmodium had more oxidative damage to lipids in their red blood cells. Our results support the idea that haemosporidian infections impair the oxidative balance of blackcaps during autumn migration, which could impair individual performance and ultimately fitness, with increased population impacts during the first year of life when a larger proportion of individuals migrate coinfected with harmful Plasmodium parasites.