It was 180 years ago this month when a young Charles Darwin first stepped onto the Galápagos Islands. He would soon come to realize that he had landed in a place of many curiosities—a laboratory cut off from the rest of the world, with creatures that had evolved faster than those on the mainland, sometimes in ways that could only be described as bizarre.
Nearly two centuries later, Darwin’s discoveries about evolution are abundantly confirmed by scientists studying species on other remote islands. J. Albert Uy, the Aresty Chair in Tropical Ecology at the University of Miami, is one of them. He coauthored a recent paper in The Auk: Ornithological Advances about variations among birds on the Solomon Islands, a chain of hundreds of isles northeast of Australia. “Typically the top predators are missing and the competition is relaxed, so you get all of these trajectories that aren’t possible on a crowded continent,” he says of islands. These “trajectories” include darker colors, loss of flight, mega-growth, shrinkage, and docile behavior. Here are some examples.
On small islands, even the most colorful birds, like Bananaquits in the Caribbean and White-winged Fairy-wrens off the western coast of Australia, occasionally go dark. (Mammals, reptiles, and insects exhibit this trait, too.) In some cases, the extra melanin is thought to help them regulate their body temperatures or blend in with their surroundings. As Uy points out, melanin also makes feathers sturdier, which is especially advantageous on islands, since birds there are more exposed to the elements. “They can withstand not just physical damage better,” he says, “but also feather-degrading microbes better.”
Uy has spent nearly a decade studying the Chestnut-bellied Monarch, a bluebird-sized songbird endemic to the Solomon Islands. His research uncovered another characteristic of dark-feathered individuals—aggression. In 2009 Uy and his team observed that pure-black monarchs, a variant of the species, greet threats from other monarchs with more hostility than chestnut-bellied ones. The group’s latest research further reveals that smaller islands in the Solomons have a higher percentage of black birds than larger islands. The takeaway: Because small islands have such a limited supply of breeding territories, the darker, more aggressive birds are successful there.
To get to an island, a bird’s going to use its wings. But once it lands there, it might find few predators. Which is why, over the span of many generations, species like the Galápagos’ Flightless Cormorant, the Guam Rail, and New Zealand’s Kakapo have lost the ability to fly. “It’s really metabolically expensive for birds to maintain their flight muscles,” says Alison G. Boyer, an ecology professor at the University of Tennessee. Without them, “[they] can put more of their energy toward reproduction, or maintaining their own body, or growing a little bit faster or bigger.”
Unfortunately, flightlessness becomes a major disadvantage once humans arrive, especially when they bring rats, cats, and livestock with them. In a 2013 paper, Boyer estimated that human colonization of the Pacific islands caused the extinction of nearly 1,000 bird species—and that doesn’t even include songbirds or seabirds. She also found that flightless species were 33 times more likely to go extinct than those that get airborne.
When birds fly, they try to keep it light. Even the heaviest of the aerial species, the Kori and Great bustards, are just 40 pounds—about the weight of a five-gallon jug of water. But when a bird no longer needs its wings, the pounds can start to pile on. New Zealand’s extinct moas, for instance, grew to 12 feet tall and more than 500 pounds; they were hunted to extinction around 1400 A.D. The elephant birds of Madagascar, also extinct, were mammoths as well. (Since moas and elephant birds didn’t have any large mammals around them, they moved into the megafauna niche occupied by four-legged grazers on the mainland.) Among birds that still exist, Boyer says, perhaps the best example of gigantism is the Takahē of New Zealand’s South Island. Though it averages only about six pounds, this species is believed to have evolved from much smaller ancestors to become the world’s largest rail.
No one knows why some island species get so enormous. One conjecture holds that birds with bigger bodies can ferment food in their guts the way cows do, allowing them to eat harder-to-digest plants. Other theories suggest that larger creatures usually live longer, can survive harsher weather, and require less food pound-for-pound because of their slower metabolisms.
The general rule of island evolution is that big animals get smaller and small animals get bigger. This trend is most commonly seen in mammals: To account for the limited resources on islands, the key deer of the Florida Keys, the extinct pygmy hippos of Madagascar, the extinct dwarf elephants of various Mediterranean islands, and even the so-called “hobbits” of Flores, an Indonesian island, all became miniaturized versions of their mainland relatives.
Because most birds are tiny, avian dwarfism is virtually unheard of. The exception: two extinct types of emus that lived off the coast of Australia. “The situation of an emu miniaturizing on an island is really, really unique,” Boyer says. “Emus are already very atypical for birds, so you can basically use the mammalian hypothesis for dwarfism to explain why they do the same thing.”
Back in 1835, Darwin was amazed at the tameness of the species on the Galápagos. He sat on the backs of giant tortoises, pulled the tails of marine iguanas, and even had birds land on his hands. In one case, he pointed his gun at a hawk and used the muzzle to push it off its perch. “A gun is here almost superfluous,” Darwin wrote. Other early explorers shared similar stories from other island chains, including Hawaii and New Zealand.
These animals could afford to be oblivious because they had very few predators—that is, until humans showed up and started killing them. “They could not evolve fast enough to regain their defense mechanisms,” Boyer says. The Dodo, which lived on Mauritius, is a classic and tragic example of this. Not only was it tame; it was also flightless, large, and easy to find. “People could essentially just walk up to them and pick them up or hit them on the head,” Boyer says. By the mid-1600s, just a few decades after Europeans colonized Mauritius, the Dodo was extinct.