My own experience here in Kenya has been limited but I will say that I have found honeybees a very serious competitor for cavities be they man made or not.  I do think its worth finding out what the optimal honeybee cavity is (I believe for instance that honeybees like to go in then up into a larger space above) and then making a cavity that does not always meet those requirements.  In the past year I have put up two barn owl boxes (amercian design), one in Langata and one at Burguret. Both were promptly occupied by bees.  If I was to try again I would change the design so that the entrance was high on the box and the birds hop down. I might look for other changes too that would discourage the bees.  I have had luck with starlings and barbets with pieces of giant bamboo with a drilled entrance hole but even some of these (despite being very small) were occupied by bees.

Cheers, Jamie

PS Mpala has had success with hornbills (red-billeds and von derdeckens) in boxes too