Modern birds (Neornithes) have a reproductive system that is highly modified compared to other amniotes. They are the only amniotes with a single functional ovary and oviduct (although within Aves there are a few exceptions, such as the Kiwi bird). I described nine fossil birds from the Jehol preserving remnants of the ovary. These specimens indicate that the loss of the right ovary occurred very early in avian evolution.
Read more:
https://www.nature.com/news/exquisite-bird-fossils-reveal-egg-producing-ovary-1.12616
https://www.sciencemag.org/news/2013/03/early-bird-loses-ovary
Link to article:
https://www.nature.com/articles/nature11985