just to add my thoughts here.

the first is that we see these birds in flocks more often than not. flock size is often >20 but the 36 over our Iringa house last Nov was exceptional.

when on SW Kilimanjaro they often occurred as low as 1300m (our house), esp during periods of low cloud. flocks also quite often seen over Arusha town but I don't recall any over Moshi town which is quite a bit lower.

these flocks are dense, not at all like Palm Swifts, the flight is quite different, almost fluttering and we often hear their clicking calls when flocks are low overhead.

they move off the Eastern Arc mts but few records (very few observers !!). In particular I recall a flock over Mikumi town at 550m

I attach a quite recent Tanzania Atlas map.

Neil