channel billed cuckoos breeding

I have been wondering why these birds make these unbelievablly loud noises. Firstly the gigantic AWWK! is heard always if it makes its noise but more rare is the strange steam train end note only heard occasionally. lately, as the mother has layed its eggs it does not make the end not any more but the chicks do. Is the end noise a contact call, breeding call or a call in agression?

Also, the other day I think I witnessed the channel billed cuckoo migrating north. Is this the usuall time for this?

Thanks