Firstly, make no mistake sexing the Owl Finch is a very difficult task. Ideally you need to spend some time watching them together.

There are slight differences in their body markings and colour. The males bottom black bar across his chest is ever so slightly wider than the female as seen in the photo above. The male is on the left. Secondly the white chest feathers on the hen are slightly off white compared to the male. This is why it is important to watch them in a group to compare these two things with each Owl Finch.

Also the black does not reach as far back behind the beak on the female compared to the male. Even then it is still very hard, as they don't stand still for too long. If in doubt the best thing to do is to obtain several Owl Finches and let them pair up on their own.

When I bought my pair it took about an hour of catching and releasing several finches until I was satisfied that they might be a pair. Fortunately for me they were a pair!