The birds with the longer beaks will be fittest in the environment post-drought because they will be able to gather resources (food) hence survive. This will enable these bords to reach reproductive again and pass down the genes. It is important to note that fitness in nature is determined by the capacity to reproduce.
Natural selection will change the allelic frequency such that the allele for long neck will be more in the population while that of short beaks will reduce. This is because short beaks trait will not be able to reach reproductive age (due to starvation) because of the incapacity to gather food resource hence their genes will keep reducing in the subsequent generations.
