Hello Colin,
I was part of a team who spent sometimes with nest boxes in several localities in Kenya between 2006 and 2011. Our target were Starlings but we ended up having an array of occupants: Bush-babies, Mongoose (single record in Mpala with 2 babies), birds (mainly hornbills, starlings, roller, sparrows and doves), and majority were invertebrates (ants, bees, wasps, spiders, etc).
Our study, whose original objectives changed on the way, concluded that nest boxes were an important resource as a wildlife refuge in the drier/savanna habitats (Lewa, Samburu, Naivasha and Mpala) - where competition was high - than in forested habitats (Aberdares, Mt. Kenya and Kakamega). Competition for occupation was stiff in dry areas where we think this could be due to lack or shortage of wild cavities unlike in forested areas where these occur in their millions.
Please let me know if anyone is interested in any or both of our two papers, "Interaction between distant taxa in the use of tree cavities in African ecosystems: a study using nestboxes" and "Interphyletic
relationships in the use of nesting cavities: mutualism, competition and amensalism among
hymenopterans and vertebrates".
On design, we had 3 models all of were easily occupied by any of our visitors. When wildlife competed for these resources, our greatest problem/enemy to our study were human beings who stole the boxes either with or without honey (others took the bees colonies home to 'fertilize' their hives or just wanted to have somewhere to keep valuables!).
Wamiti.