This is only true if coyotes have uninterrupted access. They're just as much spree killers as dogs are - they just come back and get the bodies as long as nothing spooks them off.
Coyotes can get interrupted. They kill as many as they can, and then start retrieving the bodies. If something happens in that process, they'll leave a bunch behind. That something could be the family going outside and discovering the birds.
My coyote attack was like that - I walked out to see...