Galapagos mystery solved with barn owl pellets

The mystery began when Charles Darwin visited the Galapagos Islands in 1835, collecting specimens of birds and mammals on Floreana and other islands in the archipelago. Since Darwin’s visit, other scientists and expeditions have tried to collect many of the same species, but without success. For example, Darwin and others on the 1835 expedition collected 8 specimens of a Large-Beaked Ground Finch on Floreana Island. None can be found there today. What happened to them? Did Darwin mislabel his specimens, or have the mysterious birds evolved into different species since 1835?

The search for answers to this and other mysteries of the Galapagos Islands began in earnest with the foundation of the Galapagos National Park and the Charles Darwin Research Station on Santa Cruz Island in 1958. In 1977, Dr. S. Dillon Ripley, then secretary of the Smithsonian Institution, visited the Galapagos and was determined to increase the Smithsonian’s research and conservation efforts there. He turned to his colleague Dr. Storrs Olson, a curator of birds at the Smithsonian, for ideas. Olson suggested that David Steadman visit the islands. Steadman was a research assistant who shared Olsen’s interest in bird fossil research on young volcanic islands. Olson had found bird fossils in caves on Hawaii and other volcanic islands, even though most experts considered such islands poor places for fossil hunting.

The mystery began when Charles Darwin visited the Galapagos Islands in 1835, collecting specimens of birds and mammals on Floreana and other islands in the archipelago. Since Darwin’s visit, other scientists and expeditions have tried to collect many of the same species, but without success. For example, Darwin and others on the 1835 expedition collected 8 specimens of a Large-Beaked Ground Finch on Floreana Island. None can be found there today. What happened to them? Did Darwin mislabel his specimens, or have the mysterious birds evolved into different species since 1835?

The search for answers to this and other mysteries of the Galapagos Islands began in earnest with the foundation of the Galapagos National Park and the Charles Darwin Research Station on Santa Cruz Island in 1958. In 1977, Dr. S. Dillon Ripley, then secretary of the Smithsonian Institution, visited the Galapagos and was determined to increase the Smithsonian’s research and conservation efforts there. He turned to his colleague Dr. Storrs Olson, a curator of birds at the Smithsonian, for ideas. Olson suggested that David Steadman visit the islands. Steadman was a research assistant who shared Olsen’s interest in bird fossil research on young volcanic islands. Olson had found bird fossils in caves on Hawaii and other volcanic islands, even though most experts considered such islands poor places for fossil hunting.

Enter the Barn Owl. Galapagos Barn Owls roost and nest on ledges inside collapsed lava tubes. Although rodents are their preferred prey, these opportunistic predators also prey upon reptiles and other birds. In fact, the way they eat and digest their prey has a lot to do with the presence of bird bones in the tubes. Barn Owls swallow their prey whole or in large chunks. They digest the soft tissues and regurgitate the indigestible parts of the prey such as scales, bones, feathers, and fur as walnut-sized pellets. In time, only the bones and skulls remain, and eventually they become fossils. Barn Owls typically regurgitate pellets close to their nests and roosting places, which explains the presence of bird bones in the lava tubes.

In a lava tube on Floreana Island, Steadman found the fossilized bones of a Large-Beaked Ground Finch identical in size to those found by Darwin. Here was proof that the mystery bird had indeed lived on Floreana, but what happened to it after Darwin sailed away? Did Barn Owls eat the entire population to extinction?

Darwin unwittingly provided a hint when wrote that a penal colony had been built on the island just before his visit. Goats and donkeys escaped from the colony and became feral (wild). Floreana Island is arid, so the feral animals learned to kick over and break open the prickly pear cactus to get at its inner pulp, a ready source of food and water. The Floreana prickly pear cactus (which is extremely rare today) has the largest seeds of the Galapagos cacti, and the Large-Beaked Ground Finch had a massive beak capable of cracking them. When the cactus became scarce, the finches were in mortal danger. They were so well adapted to eating its seeds that they could not eat other food. Consequently, the population of finches declined until the species became extinct.

Other introduced species also caused problems for the native species of the Galapagos. Rats unintentionally introduced to the islands from ships (including the HMS Beagle) were much more aggressive than the native cotton rats and mice. They either out competed the native species or introduced diseases for which the natives had no immunity. The Floreana Mockingbird lived on Floreana when Darwin visited the island, but none can be found there today. Cats and other animals introduced by settlers are thought to have driven it away. Similar mysteries occurred on other islands in the archipelago, and Steadman’s research has suggested solutions to many of them. However, Barn Owl pellets, the humble byproducts of digestion, were the key to solving the mystery of Darwin’s Large-Beaked Ground Finch.