Island Birds and Dark Plumage, Unknown Links Revealed

Birders who have traveled to the islands of the Caribbean and the Galapagos in the New World are likely aware that some of the endemic species there, namely bullfinches and the famous “Galapagos” finches, are a decidedly dusky hue compared to birds on the mainland. This characteristic bears out in a number of island specific species groups, and a study recently published in The Auk: Ornithological Advances might offer some reasons why. Dark birds are more aggressive birds, it turns out, and in a place where space is limited as on a small island, that characteristic means a lot

The subjects of the study were Chestnut-bellied Monarchs, Monarcha castaneiventris, a flycatcher of the family Monarchidae endemic to the Solomon Islands.While it’s been known for some time that evolution on islands tends to select for bizarre traits unknown on larger land masses (like size extremes or, in birds, flightlessness), tendency towards melanism is also a trait seen more often on restricted island birds. And in the case of these monarchs, that means more than just abnormal colors.

Studies in mammals and fish have found a genetic link between melanism and aggressive behavior, and [researchers Albert] Uy and [Luis] 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. Previous experiments with other Monarcha castaneiventris subspecies using taxidermied birds and recorded songs have shown that melanic birds react more aggressively than their chestnut-bellied counterparts when they perceive a threat to their territory.

And more, the smaller the island, the higher the proportion of dark birds to normal-colored birds, suggesting that the gene that acts on melanin production also acts on hormones that increase aggressiveness in individual birds.  And as mentioned above, that means a lot when space is scarce and high-quality territories are few.

For more on this interesting look at island evolution, see the AOU’s publications blog.

Interestingly, Chestnut-belled Monarch has been the subject of another recent study that also offered insight into evolution of plumage characteristics that found that a single gene determined whether a bird was one of two subspecies, that could interbreed but generally did not regard each other as potential mates. Taken together, one can see that there are some interesting things going on with this species.

It’s yet another example of islands being the laboratories of evolution, and birds continue to be wonderful examples of it.

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