@Kitsafari I think you are right about the fiscal but not about Mackinnon's
Mackinnon’s shrike Lanius mackinnoni is an entirely equatorial Central African species I saw one in Gabon they range from the west coast in a narrow band across Central Africa to the far west of Kenya, so its distribution completely rules it out and it's also grey on the back and the top of the head.
Although there’s quite a difference in the size of the white supercilliums on these two birds I would suggest that they are in fact the same species and that they are the south-western form of the common fiscal Lanius collaris subcoronatus. I’ve not been to Tswalu but I would guess that this subspecies should occur there. The only other fiscal that has a black head with a white supercillium like this and that does look very similar is the Uhehe fiscal but this is endemic to the Eastern Arc Mountains of Tanzania. So I think that common fiscal is the only option in both cases.