70s Horror Movie: abandoned Baby turns serial killer

My wife has a vague memory of a horror/thriller movie from the seventies: a woman, a pale, straight-haired blonde, leaves a baby with a family in a park. The family takes the baby home and raises it as theirs. Years go by. The kid starts bumping off family members one by one. She remembers “The Cookoo Bird” being specifically mentioned (which this movie is derived from).

The Mrs. Thinks she s seen the straight-haired blonde in some other films in that period, but she doesn’t remember seeing any of the other actors in other stuff.

Does this movie sound familiar to anybody?
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If the pale, straight-haired blonde was Angela Pleasence, your movie is The Godsend.

You nailed it. The Mrs. Says instantly recognized a picture of the actress, and when I mentioned the name of the movie, she confirmed it.

Thank you very much!

Aw, I thought it might have been Evil Roy Slade.