The chestnut-bellied monarch is a small bird in the flycatcher family with dark blue-black feathers everywhere except, as its name suggests, its light brown belly. If you hop around the South Pacific islands that these birds call home, though, you’ll see that not all monarchs conform to this color scheme. Some of the birds are black all over and don’t really earn their name.

Black coloration, or melanism, in animals is pretty common on islands, and has been seen in plenty of species of bugs, birds, reptiles, and spiders. In all but a few cases, though, scientists haven’t been able to figure out why that is, especially when melanistic animals live right alongside their lighter-colored counterparts. The Solomon Islands, where the chestnut-bellied monarchs live, are packed close together. The whole archipelago is only around 18 miles wide and many islands are less than a mile from each other, so the birds can easily fly from island to island and spread their genes around. Yet, says biologist J. Albert Uy, "the color forms are essentially fixed within islands.” Because the same all-black coloration has evolved independently on different islands and hasn’t faded away from the gene pool, Uy suspected there was a good reason for having it.

To find out what that was, he and biologist Luis Vargas-Castro visited 13 different islands in the archipelago and went looking for monarchs, classifying each one they found as either “chestnut-bellied,” “mostly melanic,” or “melanic.” Some islands had only chestnut-bellied birds. On others, as many as one-third of the birds they saw were black. 

Because the islands are so close together, the habitats, climate, food, and predators they offer are very similar, and Uy and Vargas-Castro didn’t see any variation in those factors that could explain the different rates of melanism. They did, however, see a clear pattern in their survey results: the smaller the island, the higher the frequency of black birds. They think it's the size of the different islands that drives the variations in color, in a roundabout way. 

In past research with a monarch subspecies, Uy and Vargas-Castro found that darker birds were more aggressive, and other researchers have noted the same correlation in different animals, suggesting there might be a genetic link between the two traits. Chestnut-bellied monarchs are socially monogamous, and mating pairs of birds defend their breeding and nesting territory from other couples. Because there’s less space to go around on the smaller islands, the researchers believe that natural selection may favor aggression, while indirectly favoring melanism. The tough birds that can capture and keep territory have a breeding advantage and make more little aggro monarchs, and the all-black coloration becomes more common as a side effect.