Much of Namibia is very dry, and thus hasn't the variation in resident birds that you might find in lusher environments. However, many of those dry-country birds have restricted distributions, and so are endemic, or close to being so. Further, where Namibia's drier interior borders on to a wetter area, as within Mahango National Park, the species count shoots up.
In addition to its residents, Namibia receives many migrants. In September and October the Palaearctic migrants appear (ie: those that come from the northern hemisphere – normally Europe), and they remain until around April or May. This is also the peak time to see the intra-African migrants, which come from further north in Africa.
The coastal wetland sites, most notably around Walvis Bay and Sandwich Harbour, receive visits from many migrating species, as well as seabird species that aren't normally seen in the interior of southern Africa. So visits including the Caprivi and the coast, as well as the country's interior, make Namibia an excellent and varied destination for birding trips.
Inevitably the rains from December to around April see an explosion in the availability of most birds' food: seeds, fruits and insects. Hence this is the prime time for birds to nest, even if it is also the most difficult time to visit the more remote areas of the country.