High-pathogenicity avian influenza (HPAI) viruses emerged from low-pathogenicity avian influenza (LPAI) viruses in Pennsylvania (1983–84), Mexico (1994–95), and Italy (1999–2000). Here we focus on the question of why the HPAI virus supersedes the LPAI virus, once it has appeared during the epidemic. To study this, we used an experimental model in chickens that enabled us to estimate the reproduction ratio (R0). Using this model, we determined the R0 of the A/Chicken/Pennsylvania/21525/83 (LPAI) and of the A/Chicken/Pennsylvania/1370/83 (HPAI). Comparing the R0 of both viruses, we concluded that the R0 of the HPAI virus is significantly higher than the R0 of the LPAI.