Nonspecific constitutive immunity is negatively associated with basal metabolic rate at early phase of territorial competition in males of great tit

Abstract During critical phases of life, animals must adjust their allocation of energy among vital functions, including those associated with immune defense. In the Moscow region, we examined 37 wintering males of great tit (Parus major) to evaluate the magnitude and direction of the relationship between nonspecific constitutive immunity (NCI) and two traits reflecting energy expenditure and balance. The first, basal metabolic rate (BMR), represents the energetic cost of self-maintenance in endotherms and could serve as an indicator of maximum sustainable work output. The second, body mass, acts as a proxy for a bird’s current physical condition. We hypothesized that under the harsh midwinter conditions, when males begin to exhibit territorial behavior, a trade-off would likely emerge between their costs of maintaining NCI and BMR during the territory partitioning phase. We used a hemolysis-hemagglutination assay to assess NCI and flow-through respirometry to estimate BMR. Our results revealed an age- and condition-dependent relationship between body mass and NCI, alongside a consistently negative relationship between NCI and BMR across age groups and years. This negative relationship indicates a trade-off between energy allocated to self-maintenance and NCI. Previously, we demonstrated that an improved social and territorial status in male great tits was associated with increased energy expenditure, either through elevated BMR or higher daily energy expenditure. We propose that the energetic cost of NCI arises indirectly due to hormonal shifts associated with changes in social status. Our findings underscore the complex interplay between physiological and ecological pressures in shaping life-history strategies.