Final answer:
Birds with small, thin beaks are said to evolve larger, broader beaks through the process of adaptive radiation and natural selection, enabling them to eat larger seeds and adapt to their environment.
Step-by-step explanation:
The concept you're asking about is an example of evolution and natural selection. The miraculous change you refer to is the evolutionary adaptation of birds' beaks to their environment. In Birds of a different feather, it is the process of adaptive radiation that leads to birds with small, thin beaks evolving over generations to have larger, broader beaks, enabling them to eat larger seeds.
As Charles Darwin observed, finch species have varying beak shapes, which are attributed to adaptations over time that allowed the finches to exploit different food sources. This explains why certain species, such as seed-eating birds, possess thicker and stronger beaks that are well-suited to cracking hard nuts, while others like nectar-eating birds have long beaks tailored for reaching the nectar inside flowers.
Observations of differences in beak shapes across finch species demonstrate evolutionary changes in response to the availability of specific food sources, leading to each species developing a beak best suited to its food source and niche.