How does the variety of birds beaks support the theory of natural selection?

Every bird have different beaks. By natural selection their beaks define what kind of food they can eat and whether or not they can survive on an island. If you did a lab on beaks and finches, you had to use some of the objects as a beak and collect as many beans as you can.
Example: There is an island filled with insects
A finch that has a beak in which they digest insects vs A finch that has a beak that digests fruits. Based on this scenario the first finch will survive because it beaks defines natural selection. The second finch eats fruits and with an island full of insects no fruit is available

B] nitrogen fixationmight be wrong, so correct me.want extra on your answers? just go on my account and go on questions and answer, or ask in the comments!