According to the concept of "survival of the fittest," if a population of birds eats worms that they find deep under ground by using their beaks, which members of the bird population would be more fit?

The fittest would be the bird with the longest beak: such a beak would allow this bird to get more worms that the other birds, since it could catch worms deeper in the ground. It would be stronger and could have more offspring, which would also likely have long beaks.