There was a flock of swifts (Bradfield's) wheeling around on the outskirts of Windhoek, as the sun was going down, plus some other nice little songbirds and weavers. There were also a few of these, which were lower than the swifts, but I could not get a clear snap of them. Sand martin perhaps? I could not however see if they had white bellies.
https://www.flickr.com/photos/41442363@N08/25506157828/in/dateposted-public/
https://www.flickr.com/photos/41442363@N08/25506157828/in/dateposted-public/