The immune system is essential for survival and reproduction by animals living in environments full of pathogens and parasites. However, the immune system competes with other physiological functions and behavior for energy and nutrients. In light of such trade-offs, immune defenses in animals are shaped by their broader ecology. In general, immune defenses can be broken into two categories: innate and adaptive. To date, ecological immunologists have emphasized the value of innate defenses over adaptive ones, since the former serve as an organism’s first line of defense. Additionally, some innate defenses are relatively consistent over time in the face of environmental changes. Targeting innate humoral immune defenses, the hemolysis–hemagglutination (HLHA) assay has two quantifiable endpoints: lysis (HL), which is a function of complement, and agglutination (HA), which is a function of natural antibodies (NAbs). In our systematic review, we investigated whether life history characteristics and other ecological traits predict HL and HA values in birds. We collected HL and HA values of 108 bird species from all peer-reviewed empirical studies that cite the original methodological publication (from 2005; n=76). We also collected from the literature data on life-history traits (clutch size, body mass, and lifespan), behavioral and ecological factors (food type, social structure, migratory strategy, and habitat type), and annual cycle stages (breeding, migration, or overwintering) of the study populations. Our results contribute to the understanding of the drivers of immunological diversity.