A major contributing factor for the reduction in the number of medium ground finches on daphne major island

The correct answer to this would be:

"inability of many medium ground finches to feed on large, hard seeds"

The medium ground finches feed on seeds. However, the birds have two variations in the bill shape: some birds have wide, deep bills while others have thinner bills. The large-billed birds feed more proficiently on large, hard seeds, while the smaller billed birds feed more proficiently on small, soft seeds. The reduction in number started in 1977, when a drought period altered vegetation on the island. Due to this event, the number of seeds declined dramatically; the decline in small, soft seeds was larger than the decline in large, hard seeds. The large-billed birds were able to endure better than the small-billed birds thus resulting in decline in number.