How does the shape of a birds beak help it survive in its environment?

In natural selection, those best suited to their environment are going to survive and pass on their traits to future generations. When birds have different shaped beaks, some are going to be better fit to get food from flowers. These birds that get more food survive more often, and have a better chance to pass on traits to their offspring. Consequently, there are more offspring with that specific trait (beak size). The population begins to shift over time towards the favorable trait.