The Silent Scream

I biked to the Kumeras today and almost as I got off the bike, this shag flew past and sat about 3 metres away. Nothing worried it - cars going past, me getting closer and more comfortable - it just carried on with its usual spread the wings to dry routine. Several times while we sat together on our respective rocks, the shag opened it's mouth very wide in what looked like a long silent scream. I wondered if anyone knows the reason for this behaviour. Is it regurgitating? Or yawning with boredom? Or a warning signal?