What differences did Charles Darwin notice among Galapagos mockingbirds?

During his visit to the Galapagos Islands in 1835, Charles Darwin observed that the mockingbirds differed across four of the islands he visited.1

Unlike his collection of other birds, which he failed to label by island, Darwin carefully tagged his mockingbird specimens according to the specific island where they were found. These observations, along with the birds' resemblance to species he had seen on the South American mainland, raised significant questions for him regarding species distribution and whether each island's mockingbirds represented naturally produced varieties. These findings became a key part of the evidence he later used to develop his theory of evolution through natural selection.12