Animal populations on islands tend to develop unusual traits over time, becoming big (like Galapagos tortoises) or small (like extinct dwarf elephants), or losing the ability to fly (as the flightless parrots of New Zealand). One less-studied pattern of evolution on islands is the tendency for animal populations to develop "melanism"--that is, dark or black coloration. Dr. J. Albert Uy and Dr. Luis Vargas-Castro of the University of Miami found an ideal species in which to study this phenomenon: the chestnut-bellied monarch (Monarcha castaneiventris) flycatcher, a bird found in the Solomon Islands. Most of these birds have the chestnut belly suggested by their name, but in the subspecies found in the Russell Islands, a few all-black birds coexist with the chestnut-bellied majority. After visiting 13 islands of varying sizes to survey their chestnut-bellied monarch populations, Dr. Uy and Dr. Vargas-Castro confirm, in a new paper published online on July 22, 2015 in an open-access article in The Auk: Ornithological Advances, that island size predicts the frequency of melanic birds, with populations on smaller islands including more dark individuals. The article is titled “Island Size Predicts the Frequency of Melanic Birds in the Color-Polymorphic Flycatcher Monarcha castaneiventris of the Solomon Islands." Because the pattern is repeated on island after island, it is very unlikely to have developed through random chance; instead, dark coloration most likely provides some sort of benefit to birds on small islands. Studies in mammals and fish have found a genetic link between melanism and aggressive behavior, and Dr. Uy and Dr. Vargas-Castro speculate that the limited space available on smaller islands makes competition for breeding territories more intense, giving an advantage to the most aggressive individuals.
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