The correct answer is option 4 - Ovary. Birds usually have a single ovary.

Option 1 - Kidney: Birds typically have two kidneys, not just one.

Option 2 - Lung: Birds have a unique respiratory system which includes air sacs and a complex network of tubes called bronchi, so they have more than just one lung.

Option 3 - Testis: This option is incorrect for birds. Testes are a male reproductive organ, and while birds do have testes, they do not have a single one. In fact, most male birds have two testes.

Option 4 - Ovary: This is the correct answer. Female birds have a single ovary, which is where eggs are produced. Unlike mammals, where females typically have two ovaries, birds only have one.