I've received these interesting images from Ken Behrens, an experienced tour guide (for Tropical Birding) and author (great books!) who suspects the bird involved is a Speckled x White-headed Mousebird. The images were taken close to the Lark Plains in northern Tanzania.

I have placed the two images together; on the left-hand image, the bird in question is the one that is partially hidden in the background next to an obvious White-headed. It certainly shows a combination of features and I wonder if anyone has any knowledge, or experience, of such hybrids as I cannot locate any references in the available literature.

We welcome your thoughts on this individual.

Many thanks

Adam