What came first: the genetic variation of different beaks or natural selection acting upon them?

I believe the situation is something like the birds with shorter/longer beaks (depending on the situation) survive.

To answer the question, we have to understand the meaning of natural selection. natural selection is where the organisms with more favourable characteristics survive and reproduce and pass on the genes to offsprings while some of the organisms with less favourable characteristics die, so that there would be more organisms with that favorable characteristic in the population.

So in order to have natural selection happen, Birds must have different lengths of beaks, in order to make it happen, genetic variation must occur. if all the birds have the same length of beak, the either all survive or die, which is not the definition of natural selection.

Therefore, your answer should be genetic variation.