i dont know exactly what characteristic it would be
Explanation:
the variation of the beak shape was because the islands had diffrences to them. so on some of the islands the nuts would be bigger or smaller. The birds had to adapt to this so their beaks changed when you looked at the diffrent birds on the diffrent islands. The finches would have big beaks on one island because the nuts were bigger and they need food, but on another island the nuts were smaller so the finches on that island would have a smaller and more narrow beak. so bassiaclly the diffrences were because the birds adapted over time so they could use their beaks to properly eat.
