Darwin noticed several types of small brown birds on the islands with beaks of different shapes. He thought that some were wrens, some were warblers, and some were blackbirds. ... the little brown birds that Darwin thought were wrens, warblers, and blackbirds were actually all species of finches! They, too, were found nowhere else, though they resembled a South American finch species ... Darwin was stunned by these discoveries. He began to wonder whether different Galápagos species might have evolved from South American ancestors.It's a pretty story, but it turns out that, like so much else we were taught about evolution, it isn't true. According to a Harvard historian of science named Frank Sulloway, Darwin was almost indifferent to the birds he encountered on the Galapagos Islands. Sulloway is quoted by Alberto A. Martinez in his book Science Secrets: The Truth about Darwin's Finches, Einstein's Wife, and Other Myths. Martinez writes:
He spent years filling notebooks with ideas about species and evolution. ... Once Darwin learned that the birds were all finches, he hypothesized that they had descended from a common ancestor. Darwin noted that several finch species have beaks of very different sizes and shapes. Each species uses its beak like a specialized tool to pick up and handle its food... Darwin proposed that natural selection had shaped the beaks of different bird populations as they became adapted to eat different foods.
Many old books claim that when Charles Darwin visited the Galápagos Islands, he was inspired to think about evolution by seeing variations in finches' beaks. ... Allegedly, he found that each species of finch belonged to a particular island and had developed distinct feeding habits that matched their evolving beaks, for cracking small or big seeds or for eating insects. That's what many people still think, and so, one of the most widely reproduced pictures in history is that of Darwin's finches.There's more on this at Evolution News and Views. Like the Peppered moth story, the Recapitulation theory, Junk DNA, and so much else still found in textbooks even though they're obsolete and/or discredited, it persists for years in edition after edition because it makes for such a fascinating tale. After all, the claims don't have to be true, exactly, as long as they help students to understand the theory, or at least that seems to be the justification for continuing to teach these myths and falsehoods to our kids.
However, in sterling historical studies, Frank J. Sulloway of Harvard University showed that, really, Darwin was hardly influenced by finches and scarcely observed their feeding habits.
He did not correlate their diets and beaks; in fact, Darwin collected too few specimens to determine whether any finch species was unique to each island. He did not even keep track of where he picked up every specimen. Really, no finch species was unique to any one island. Unfortunately, some teachers and writers remain unaware of Sulloway's historical findings.