Interesting question. Presumably, like most migrant cuckoos they call for social or sexual reasons when they arrive at their breeding areas. But I don’t think that is the question being asked. Which raises to me the idea that yes they are cuckoos, but in their way of feeding they are different from most cuckoos. They are mainly fruit eaters. I would think that fruit is a geographically concentrated food source, as distinct from the more evenly spread insect food source that most cuckoos use. So I propose (for consideration, with no proof I hope you understand) that maybe it is the calling related to finding food – if they want to share it that is, or maybe it provides a way to assemble in groups at places with abundant food.
Philip
