Trans-generational antibody transfer constitutes an important mechanism by which mothers may enhance offspring resistance to pathogens. Thus, differential antibody deposition may potentially allow a female to differentiate offspring performance. Here, we examined whether maternal immunization with sheep red blood cells (SRBC) prior to egg laying affects sex-specific yolk antibody transfer and sexspecific offspring performance in zebra finches (Taeniopygia guttata). We showed that immunized mothers deposit anti-SRBC antibodies into the eggs depending on embryo sex and laying order, and that maternal exposure to SRBC positively affects the body size of female, but not male offspring. This is the first study reporting sex-specific consequences of maternal immunization on offspring performance, and suggests that antibody transfer may constitute an adaptive mechanism of maternal favouritism.