A common noun is a name for a person, place, or thing that can be found in many places and is not unique. For example, "dog", "cat", "tree", and "computer" are all common nouns.
Names of birds are not common nouns because they are specific to one type of bird. For example, "robin", "blue jay", and "sparrow" are all names of birds, but they are not common nouns because they can only be used to refer to one specific type of bird.