The term that describes the situation is called: stabilizing selection.
Unlike disruptive selection (diversifying selection) that tends to favor extreme traits when changes occur, stabilizing selection is defined as changes in population genetics in which extreme traits are not favored, rather, traits that are of intermediate variants are most favored and selected for.
Stabilizing selection can simply be described as the exact opposite of disruptive selection.
In the scenario given, traits of producing fewer starlings or more are extreme traits that are not favored and have reduced fitness. Intermediate variants of producing moderate number of starlings that have high fitness demonstrates the type of selection that is referred by the term called: stabilizing selection.
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