Alliteration is literally defined at "the occurrence of the same letter or sound at the beginning of adjacent or closely connected words." So by using alliteration he gets a rhythm into the readers head to help them understand the story.
In times of severe drought, the average beak depth increases in order to handle the limited foodsupply. With only tough seeds to eat, a large beak is needed. Due to a large beak being needed, the small and medium beaked finches died out after 5 years as they were unable to adapt to these new conditions.
