An adult bird perches on the edge of its nest. all of the baby birds open their mouths. what type of behavior is this?

The behavior that the scenario shows is innate, it is because it is already natural for the way how the baby birds responds is because they were asking or have the thought of having food as now that they saw their mother. It is a natural response of the baby bird when they are hungry and to ask when they saw their mother perching on their nest.