Black-crowned Waxbill - altitudinal wanderings?

Another interesting set of sightings from Dave up at Sabinyo Silverback lodge. (NW Rwanda adjacent to Volcanoes NP)

He mentioned the other day to me that he had seen Black-crowned Waxbill Estrilda nonnula up at the lodge - I thought this very odd as I've only recorded Black-headed/Kandt's Waxbill E. atricapilla there and the altitude 2500+m would be theoretically too high for E. nonnula.

Black-crowned Waxbill (photo by Dave Richards)

Dave got this photo of an unmistakable E. nonnula during March-June stint up at the lodge. The vent is white and the white underparts correspond with my photos (one below) of E. nonnula down in Ruhengeri (altitude 1840m). Even More interesting is that Dave saw some nonnula feeding with a loose flock of atricapilla! Something I've not come across in Rwanda at all before.

Stevenson & Fanshawe (Field Guide to the Birds of East Africa - Kenya Tanzania Uganda Rwanda Burundi) give altitudinal range for E. nonnula as 800-2200m and Zimmerman et al (Birds of Kenya & Northern Tanzania) as 1500-2000m - either way, these birds are minimum 300m above the range. Britton (Britton 1980) in "Birds of East Africa" does not refer to Rwanda but mentions Virunga Volcanoes, Kigezi in SW Uganda where E. nonnula occurs below 1600m. An altitudinal range of 800-2200m is also given.

The habitat in this case for E. nonnula per se is thus fine i.e. forest edges and clearings but has gone at least 300m above the known (from the above literature) range.

Below I've added 2 of my photos of the both E. nonnula (Ruhengeri 1840m altitude) and E. atricapilla (also lodge location at c.2520m altitude)

Black-crowned Waxbill E. nonnula photographed in Ruhengeri town (Rwanda) at 1840m altitude.

Black-headed (Kandt's) Waxbill E. atricapilla photographed in 2009 at Sabinyo Silverback Lodge at c.2520m altitude