As they call at all times of the breeding cycle I would think that the suggestion that it is to ‘pick up’ immature birds would not fully explain why they call in flight.
Regards
Greg Dr Greg. P. Clancy Ecologist and Wildlife Guide Coutts Crossing NSW
I’ve seen 25 Yellow-billed Cuckoos together – albeit on a small island (Dry Tortugas) during migration.
Nikolas