'Gargling' Cormorants

Why do Cormorants 'gargle'? I've noticed this behavior in many Cormorants for a long time. Watching their necks I've wondered, Are they singing vibrado? Gargling to sooth a sore throat? Practicing a new breathing technique? Yodeling? I finally got the answer while observing them on the log boom this day. Watch to the end to see what's really going on. The Parks Reporter