Developmental timing of signals affects information content: song complexity but not consistency reflects innate immune strategy in male song sparrows.

In short-lived animals, innate immunity is an important component of fitness and quality. Although receivers cannot generally assess a signaler's immune function directly, sexually selected displays such as birdsong may reflect past or current condition. We investigated the degree to which song complexity and consistency, thought to reflect condition over different developmental timescales, predict multiple aspects of innate immunity in male song sparrows (Melospiza melodia). We also investigated correlations among immune measures. Noncellular components of innate immunity (soluble blood proteins including natural antibody and other protective proteins) were negatively related to cellular (phagocytosis-based) components, suggesting trade-offs within innate immune protection. This pattern underscores the risk of inferring "immunocompetence" from a single metric. Song complexity, a permanent trait in this species, was positively related to noncellular relative to cellular immune components and may thus provide information as to the singer's innate immune strategy (investment in noncellular vs. cellular activity). Such a relationship could arise through shared timing of song learning and antibody repertoire development in early life. Singing consistency, thought to track variation in current condition and measured at both whole-song and syllable scales, did not predict any immune measures. Developmental timing of signals thus appears to influence their information content.