What are the birds that Darwin observed on the Galapagos Islands? What differences did he notice between them?
Your question has to do with the variation of finches that Darwin observed in the Galapagos. Although science historians have exaggerated how much time he dedicated to his writings about them, their importance cannot be understated.
The primary difference that he noticed was their beaks, specifically their size and shape. What was important was that each finch was unique to each island he visited, and that each island was unique in the habitats and food sources it offered for these birds. When Darwin analyzed the birds further, he noticed that the beaks made the birds best suited for the environment they were in, specifically the type of food available. For example, on one island, there was an abundance of nuts that would provide rich nutrients inside the shells. On this island, Darwin noted that the native finches had shorter, tougher beaks, capable of cracking those shells. On other islands, flowers were the dominant food source, and the birds had narrower, longer beaks, capable of accessing the nectar inside.
The commonality in each island is that each variation of finch was uniquely fit for survival on that island. Further, Darwin surmised that the finches were all descendants of a common ancestor on the mainland, each experiencing a type of microevolution over time upon reaching these islands.