Do birds have single or double circulatory systems?

In a single circulatory system blood is pumped from the heart to the gas exchange organ and the to the rest of the body.

In a single circulatory system low blood pressure is maintained.

Blood travels to organs more slowly.

In a double circulatory system blood is pumped from the heart to the gas exchange organ and again to the heart and from the heart the blood is pumped to the rest of the body.

In a double circulatory system high blood pressure is maintained as blood must be pumped at a faster rate to the parts of the body.

blood travels more faster to organs

Animals with single circulatory systems, e.g:Fish

Animals with double circulatory systems, e.g:Humans