The influenza A/Mallard/Pennsylvania/10218/1984 (H5N2) virus is unable to replicate in 3-wk-old immunocompetent specific-pathogen-free chickens when a dose of 5 × 10⁶ 50% egg infectious dose/ml is used. In contrast, this mallard virus shows limited replication in 3-wk-old chickens that had been previously infected at 2 days of age with, and recovered from, the immunosuppressive agent infectious bursal disease virus (IBDV; herein referred to as IBDV chickens). This limited replication in IBDV chickens allowed for the serial passage of the mallard influenza virus in chickens. After 22 passages (P22) in IBDV chickens, the resulting chicken-adapted influenza virus replicated in both immunocompetent and IBDV chickens more efficiently than the mallard influenza virus. Analysis of the outcomes of infection and the lesions caused by the two viruses at the microscopic level in a time-point study showed that the P22 virus is more virulent than the parental mallard virus in both immunocompetent and IBDV chickens. Our studies provide evidence that a previous history of IBDV infection in chickens may render them more susceptible to avian influenza virus (AIV) infections, allowing for the potential introduction of AIVs in an otherwise resistant population.