The red morph repeatedly gave a series of 3, sometimes 4, low call notes that I had long attributed to another call made by juv. birds. As you will see in the attached it is is the adult that makes this particular call. It is however not dissimilar to that of young which are a bit older. This is the call of an adult seeking it's young. This short video that the two chicks was made when two young in sight and were responding by trying to move closer despite harassment by a few small passerines.
http://www.youtube.com/watchv=NDINz4Co4YI