There are major differences in the blood circulation of fishes & birds.
Heart in fishes is 2-chambered while heart in birds is 3-chambered. In fishes the blood enters in the heart only during one whole circulation a single time while in birds it enters twice during one complete circulation. In fishes, both oxygenated & deoxygenated blood gets mixed and then circulated to the whole body, while in birds both oxygenated & deoxygenated remains separated and thus more efficient.
