Fossilized ovaries

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

https://www.smithsonianmag.com/smart-news/would-you-give-up-an-ovary-for-the-ability-to-fly-birds-might-have-3682152/

Link to article:
https://www.nature.com/articles/nature11985