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Ani­mal pop­u­la­tions on islands tend to develop weird traits over time, becom­ing big (like Gala­pa­gos tor­toises) or small (like extinct dwarf ele­phants) or los­ing the abil­ity to fly (like 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.

J. Albert Uy and Luis Vargas-​Castro of the Uni­ver­sity of Miami found an ideal species to study this phe­nom­e­non of melanism devel­op­ment on geo­graph­i­cal islands in the Chestnut-​bellied Monarch fly­catcher (Monar­cha cas­taneiven­tris), a bird found in the Solomon Islands. Most have the chest­nut belly, as their name sug­gests, 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, Uy and Vargas-​Castro con­firm in a new paper pub­lished online on 22 July in The Auk: Ornitho­log­i­cal Advances that island size pre­dicts the fre­quency of melanic birds, with pop­u­la­tions on smaller islands includ­ing more dark individuals.

Because the pat­tern is repeated on island after island, it is very unlikely to have devel­oped through ran­dom chance; instead, dark col­oration must pro­vide some sort of ben­e­fit to birds on small islands. Stud­ies in mam­mals and fish have found a genetic link between melanism and aggres­sive behav­iour, and Uy and 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.

Uy had been fas­ci­nated by Chestnut-​bellied Mon­archs ever since read­ing a descrip­tion of their plumage vari­a­tions in Ernst Mayr’s sem­i­nal book on spe­ci­a­tion, Sys­tem­at­ics and the Ori­gin of Species from a View­point of a Zool­o­gist, when he was a grad­u­ate stu­dent. “I was hooked and longed to work on the group,” he says. “I thought this would be the per­fect species to explore these ques­tions about the ecol­ogy of plumage diver­si­fi­ca­tion and the ori­gin of species, as the vari­able pop­u­la­tions of the chestnut-​bellied fly­catcher may be at dif­fer­ent stages of the spe­ci­a­tion process. It took me over a decade to finally man­age to get to the Solomons, and I’ve been work­ing on these fly­catch­ers now for nearly 10 years.”

“Pat­terns of bio­di­ver­sity on islands have always been impor­tant for under­stand­ing fun­da­men­tal prin­ci­pals in ecol­ogy and evo­lu­tion. Using the same arch­i­pel­ago that enchanted Ernst Mayr decades ago, Uy and Vargas-​Castro reveal fas­ci­nat­ing pat­terns of melanism and island size,” adds Rebecca Safran of the Uni­ver­sity of Col­orado, an expert on diver­gence between bird pop­u­la­tions who was not involved in the study. “These pat­terns add to the fun­da­men­tal impor­tance of islands as nat­ural exper­i­ments for stud­ies in biodiversity.”

Trailer — Island of Cre­ation
The research of J. Albert Uy on the Solomon Islands has been filmed and pro­duced into an hour-​long doc­u­men­tary about spe­ci­a­tion — the evo­lu­tion­ary process of species devel­op­ment. The film, Island of Cre­ation, by Day’s Edge Pro­duc­tions has been screened in April at the Cos­ford Cin­ema of the Uni­ver­sity of Miami. It will debut in July this year on the Smith­son­ian Chan­nel, which co-​produced the doc­u­men­tary. The trailer:

(Source: The Auk press release, 22.07.2015)