A very easy species to sex visually and my favourite Waxbill!

 

Cocks have a slightly more extensive black cheek patch than the hen (cock left)

 

Also very evident when viewed from the top.

 

And again in true estrildan fashion there is a difference in the ventral region, in this case the cock's whole ventral region, belly and undertail coverts are jet black compared to the hens charcoal (cock left)

Now, back to my statement as labeling them 2 separate species I will point out the differences between them. The Southern one is the Estrilda erythronotos and the Northern one is the Estrilda charmosyna with one sub-species, the Estrilda charmosyna delameri (the bird in the pics)

Note delameri is a much lighter bird with the darker red on the belly of erythronotos replaced with pink on delameri (delameri right)

Delameri (right) also has a faint whitish border to the black mask (more evident in the more colourful charmosyna which sadly never gets imported)

The difference in the two hens are even more obvious, with delameri (left) much lighter and lacking the darker ventral region as well as the pink colouration.

In this profile very little difference other than being lighter

Now the birds are very similar so I can understand why they are labelled as sub-species, but I base my two separate species theory on two foundations : 1 - The two are separated by around 1 600 km and in my book a sub-species is a bird that gradually changes over its continuous range, and 2 - The 2 species has a completely different call, the delameri having a very similar call to the Grey Waxbill