Several times already, in various locations of "Reserva Ecológica Mãe-da-Lua" and also in "Fazenda Canaã", I saw cuckoos similar to C. melacoryphus, but with a yellow mandible. This could be C. euleri or C. americanus, see figures and distribution maps in Mata et al 2006.
The main morphological difference between the latter two species, best visible in flight, is that the primaries of C. americanus are rufous, while the primaries of C. euleri are gray-brown. Until now, I have not been able to check this detail out, and for the time being, the cuckoo with the yellow mandible remains classified as Coccyzus sp.