Extra-pair fertlizations (EPFs) are common in socially monogamous passerines, but the contribution of EPFs to variance in reproductive fitness among males and to the evolution of secondary male ornaments is unclear. To understand this issue, benefits from EPFs to females should be determined. This project focuses on understanding (1) how EPFs contribute to the variance in male reproductive success, (2) the signaling function of carotenoid-based feather ornament (“good genes” indicated?) and its role in determining EP success of males, and (3) the benefits the females could obtain from EP matings (viability genes obtained?). The “good genes as heterozygosity” and “disassortative mating” hypotheses that differ in the expected contribution of EPFs to the strength of sexual selection will be evaluated. New microsatellites will be searched for to describe overall genome heterozygosity of individuals. Complex studies, combining genetic, immunological and behavioral approaches to provide complex picture.