I'm not giving up on the idea that at least one of the skulkers is a black coucal. I've seen some flashes of black and rust that seem brighter than the white-browed coucal.
One of our bird books refers to coucals as "cumbersome." They certainly do not seem very graceful. They plod along the ground awkwardly and they are not inconspicuous when trying to find cover in bushes. The black coucals have a somewhat sleeker appearance, but the white-browed coucals have a blustery look. They make a lot of rustling noises, which you'd think would attract predators. But, maybe they let their guard down in our yard because it's a relatively safe area.
Top photo: White-browed coucal from http://en.wikipedia.org/wiki/White-browed_Coucal
Bottom photo: Black coucal from http://www.cse.iitk.ac.in/~amit/birds/index.html