Explain how the beaks of the finches illustruate a structure/function relationship?

If you examine a picture of a Hawfinch (plenty are available on line) you will see that it has a short, powerful bill adapted for cracking stones of the hawthorn fruit - hence the name. Goldfinches have slender bills, evolved for extracting small seeds from thistle heads. An extreme example is provided by the various species of Crossbills, whose bills cross over at the end and are adapted for opening up the cones of pines and other conifers.