Instead of having two ovaries, birds have single ovary. It is a very significant outcome of the evolution and adaptation as well. There are proper justifications behind this. The adult birds contain a single ovary. It is an important and advantageous event for the birds as it reduces the weight of the body, especially during the breeding season. Reduction of weight is of utmost importance for the aerial mode of life. The ovary remains small and carries insignificant weight in non-breeding birds, but during the breeding season the ovary enlarges enormously and many large, heavy and yolky eggs develop in it, which increase the weight of the body. Due to the large size, and heaviness of the eggs, the birds cannot possibly afford to keep two functional ovaries in the body. Thus, loss of an ovary is a significant step in the elimination of unnecessary weight. Presence of a single ovary also helps in the safe laying of large eggs with fragile shells.