It is because they contain an abundance of chlorophyll. There is so much chlorophyll in an active leaf that the green masks other pigment colors. Light regulates chlorophyll production, so as autumn days grow shorter, less chlorophyll is produced. The decomposition rate of chlorophyll remains constant, so the green color starts to fade from leaves.
Due to the drought they would need to dig deeper for food. But their tiny beaks could not do that. So over time their beaks grew and adapted to the drought. Therefor the answer is D. They had larger beaks!
