Owls like to eat the heads of their prey.

Decapitated prey is one of the classic signs of weasel activity.

Christopher Lee

Can you relocate the bird bath to a place that would give the birds more opportunity to see trouble coming and get away?

Classic owl behavior.
Sam

Are you finding anything else other than the head like feathers, feet or gizzard? Coopers and Sharp Shinned Hawks will catch birds in backyards and eat them on site stripping off feathers and leaving some parts behind.
I hate when this happens and close my blinds and do not watch! VAL

Yesterday in my yard there was a blackbird systematically deconstructing a baby bird. When I went closer to investigate, it took off with the body and all that remained was the head.

Same situation in our porch. I found decapitated small brown birds on the same area aswell. It was 2 headless bodies of a bird and a head. Then yesterday found another dead bird on the very same spot