This figure comprises two bar diagrams plotting frequency (numbers) of birds against beak depth. The vertical axis is marked from zero to 50 in (a) and from zero to 40 in (b). Beak depth intervals, marked along each horizontal axis, are from 7.1-7.5 mm up to 11.1-11.5 mm. Part (a) shows the data for the 1977 parents: both those that were present before the drought that occurred that year, and those that survived the drought and went on to breed. Numbers were greatly reduced following the drought; for example, the frequency for the 9.1-9.5 mm beak size interval fell from 45 down to 8. Part (b) shows the data for the 1978 offspring. Vertical dashed lines mark the average beak depth values, x̄, for each of the three groups: from part (a), x̄ of birds before the drought = 9.2 mm and x̄ of the breeding survivors = 9.9.mm; from part (b), x̄ of the offspring =9.7 mm.