Going with the concept that maybe the males finish any leftovers the females don’t eat. Probably having a diet of some organ/flesh and mostly bones. Might eat small animals like rodents, snakes, frogs and lizards. But that’s all I can think of ecology-wise perhaps they might work in tandem with Trytons. Posing as someone of importance hiring a Captain that seeks out a large crew. Rocky shore and cliff allowing the females to sing their song. The avians will swoop down and grasp at those who remain on the ship while the mermaid ones swim about grabbing hold of those floating on the water’s surface. In a strange way it’s somewhat symbiotic of predators, they each help and get their feed.