We investigated whether the sexually selected forehead patch of the collared flycatcher Ficedula albicollis is an honest badge of status indicating quality expressed as immunological response. We used both manual measurements and digital measurements, the latter based on photographs. Badge-size data were collected during the mating period and during the nestling feeding period to capture trait plasticity. Concomitant with first sample collection, birds were inoculated with a novel antigen. Antibody response was strongly and positively correlated with badge expression during the mating period and with the increase in badge expression during the mating period as compared with outside this period. The results support the Hansen and Rohwer theory of coverable badges, are consistent with the immunocompetence handicap hypothesis and with the good genes model suggesting that, on a population level, the expression of secondary sexual traits should be an honest signal positively associated with traits that are beneficial for survival. The results also suggest that manual measurements of this type of secondary sexual trait are sufficiently exact.