Robert owns . . . . . . . . yellow and . . . . . . . . blue bird.

If two nouns refer to the same person or thing, article is used before the first noun only. But if the nouns refer to different persons or things, article is used before each noun. Since in the given statement, both the nouns yellow and blue refer to one bird, article will be used before the first noun only and no article will be used before the second. Also, indefinite article 'a' should be used whenever noun is unspecific. Therefore, Option C is the correct answer.