Researchers studying parental care in 20 female robins found that every female robin would place a recently collected worm into the first open mouth she saw in her nest. It appeared that the sight of an open mouth triggered the female bird's feeding behavior. To test this hypothesis, a robotic baby robin was added to every one of the 20 nests. The robot baby always had its mouth open, and the female robin always fed it first. We can conclude that _________