The graph below shows how the average beak size depth in a population of ground finch shifted during particularly wet and dry years 1977 1980 and 1982 were all drought years 1984 was a wet year During drought years all the seeds were large and tough to open This meant that birds exercised their beaks more making the beaks stronger beak depth mm 10 5 10 0 9 5 9 0 0 1977 1980 1982 1984