Different habitats support different food supplies; rich habitats can support starlings with bigger clutches. Genetics are involved. Conservative birds may be best in lean years big clutches best in good years--both will be selected for in overall population fitness
WHY DOES SURVIVAL DECREASE IN SMALL NESTS?
1) Perhaps low clutch size is a reflection of poor habitat?
2) physiology of nests--thermal advantage in cool climates for bigger broods (the babies keep each other warm)--indeed, larger nests may need less food to thermally maintain young. On the other hand, large broods may get too hot in tropics-and there are few big ones there.