It is dangerous to feed marshmallows, peanut butter, anything "sticky" to birds. Their bills can stick together, other issues, causing injury or death.
Many things. Hawks, Owls, Kestrels, other birds of prey. Cats, snakes.There are many more. Anything that's a predator AND has the ability to move fast or hover in the air, will probably eat birds.
Some birds like hawks, vultures, falcons, eagles, owls, and crows eat other birds. Some of these actually catch their prey, others are scavengers that eat dead birds.