The chestnut-bellied monarch (Monarcha castaneiventris) or chestnut-bellied monarch-flycatcher is a species of bird in the family Monarchidae. It is endemic to the Solomon Islands.

Subspecies

Five subspecies are recognised: M. c. erythrostictus has sometimes been treated as a separate species, the Bougainville monarch

Behaviour and ecology

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.