Is this a robber fly?

Is this a robber fly?

Observation - Is this a robber fly? - Southern Africa. Description: We saw several of these guys while doing a bird survey in Silvermine West (with permission from SANParks!). They mostly seemed to be resting on the heavy grass seed heads of the grass (Eragrostis sp.?) that is now very common in the open vegetation areas

We saw several of these guys while doing a bird survey in Silvermine West (with permission from SANParks!). They mostly seemed to be resting on the heavy grass seed heads of the grass (Eragrostis sp.?) that is now very common in the open vegetation areas along the footpath and the road. I think this grass is an invasive species from elsewhere in South Africa (?). Not sure if that is at all relevant to this fly, though!