Like many bush-shrikes, they have a wide vocal repertoire that includes duets in which two individuals give notes alternately in so rapid a sequence that they sound like one bird. Males probably start most duets, and their notes are mostly low-pitched whistles and/or harsh croaks; females' notes are typically higher whistles and/or "harsh tearing or rattling sounds". But although the birds' vocalizations are somewhat harsh, they are still able to create a wide range of frequencies and males provide the higher voice in certain duets.
Duets usually consist of one exchange, two or three calls in total. But up to seven exchanges have been recorded. A duet may be repeated up to 75 times, and in experiments where the birds were confronted with taped vocalizations, they could be enticed to up to 200 duets. A dozen or more duet types exist, and some seem to confer specific information, forming a Morse-code-like sort of language. Examples of typical duet sequences include hoooooo-ho-ho, hoho-u-ho, hoo-hii-hoo, haw-Weeer-haw, hoou-Weer-hoou, houhou-Weeer and bobobobo-Weeer. LOL, side splitting stuff!
The article goes into much more detail about duet calls and responses and I didn't completely understand everything but I think I get the gist of it, which is that these blighters like singing duets, or even trios, and these duets and trios all have different meanings, it's like singing in morse code. Cool eh?
Our trip to South Africa, and specifically to Kruger National Park had been amazing. The importance of protected areas such as the Park cannot be over-stated. As the human population continues to increase exponentially leaving very little room or natural habitat on this planet for other species, a few protected environments will be the only hope for the biodiversity that is otherwise being destroyed by the remorseless spread of homo-sapiens.