The Chestnut-bellied monarch (Monarcha castaneiventris) is a species of bird in the Monarchidae family. It is endemic to the Solomon Islands. In 2009, it was reported that a genetic change in some members of this species caused their colouration and songs to be different from other members of the species. As a result, members in one group did not recognize members in the other, so the two groups became reproductively isolated from each other. It was thought that over time, this could eventually lead to the creation of a new species, and that this was an example of biological evolution.