LAUSR

Abstract Migration is energetically demanding and physiologically challenging. Migrating birds, for example, need to boost their antioxidant defenses to defeat the pro‐oxidants produced during high energetic activity. The enhanced antioxidant defense possibly withdraws limited resources (e.g., energy or micronutrients) from other physiological functions, such as immune defense. Such a trade‐off might not occur outside the migration seasons or in resident individuals. Here, we investigate whether there is a negative relationship between innate immune function and antioxidant defense by sampling both migrating and resident blackbirds (Turdus merula) at the same location during the same period of the annual cycle. We show that in migrating blackbirds microbial killing capacity (BKA), an integrative measure of baseline innate immune function was negatively correlated with total nonenzymatic antioxidant capacity. In contrast, in resident conspecifics, sampled at the same time and location, these two physiological measures were not correlated. This suggests that migrating birds trade off innate immune function and antioxidant defense. Furthermore, and likely a consequence of this trade‐off, in migrant blackbirds BKA was positively correlated with oxidative damage to lipids. In resident blackbirds BKA and degree of lipid oxidation were uncorrelated. The mechanism and currencies of the supposed trade‐off are currently unknown, but energetic investments or micronutrients are likely candidates. Future experimental studies could provide more conclusive evidence for this trade‐off; yet, our results open up a new level of thinking about the physiological costs of migration.