From what I’ve seen, bitterns are one of a small number of birds that could play in the NBA based on their propensity to dunk. I’ve seen them catch voles and salamanders and snakes and frogs and, if possible, all of them get quick dunks into the pond while the bittern applies constant pressure with its large beak. It’s much too fast to be an attempt to drown the creature, it really is just a quick splash in the water, but I’m not sure what purpose it serves. This Townsend’s vole is coming up from a dunking and as I recall, got at least one more before it finally stopped wiggling and the bittern swallowed it whole.

I have seen bitterns hold snakes under the water for an extended period of time, gripping them with their bare feet, and that may have been an attempt to drown the snake, get it so cold that it would move slowly, or just keep the bitey-bits away from sensitive areas until pressure killed it.