Translation:The chicken is fast.
"Chickens are fast." is now accepted. 이/가 can also be the topic marker for emphasis though this usage is not introduced yet here.
Low level here. 닭은 would mean chickens in general. (이) specifies "the" chicken I believe.
It's referring to a single chicken. If it was 닭는 then maybe you can say it's plural, because it would be referencing chickens in general