Final answer:
Birds with short, thick bills, such as Galápagos finches and crossbills, have evolved these traits to efficiently eat hard seeds and nuts. Studies by researchers like the Grants have shown that these beak shapes are a result of natural selection and demonstrate how beak size can change in response to the availability of food sources.
Step-by-step explanation:
Birds that have evolved to have short, thick bills are typically those that eat seeds or nuts, as they require the strength of a heavier bill to crack open their food sources. An iconic example of this kind of evolution in bird beaks can be seen in Galápagos finches, specifically the medium ground finches studied by Peter and Rosemary Grant. Their extensive research on Daphne Major has shown natural selection in action, leading to changes in bill size and shape in response to the availability of different types of seeds. During drought, large-billed birds survived better due to the availability of large, hard seeds, whereas during wetter periods when smaller seeds were abundant, smaller-billed birds had a survival advantage.
Another example of a bird with a specialized bill is the red crossbill, whose beak is designed to open the cones of conifers to access the seeds inside (as shown in Fig. 3). The variety in beak shapes among bird species is a wonderful demonstration of evolution by natural selection, where birds have developed different beak types suited to their particular food sources. For instance, seed-eating birds tend to have thicker, stronger beaks to break hard seeds, while nectar-eating and insect-eating birds have evolved beaks that are better suited to their diets, such as long beaks for accessing nectar and pointed beaks for catching insects.