Of Gulliverian parrots and Lilliputian eagles: size changes on islands

This is the ninth post in the Specimen Tales series

Anand Krishnan writes ~

The biogeography of islands has long fascinated scientists. The faunal diversity in the Galapagos Islands and the Malay archipelago, respectively, provided the impetus for Darwin and Wallace to propose their theories on the evolution of species. Birds typically arrive on islands by volant colonization (a fancy way of saying they flew there), and may then evolve different forms from their mainland ancestors. The work of J. Bristol Foster, Ted Case and others has related changes in size on islands to (among other things), the presence of territoriality, the availability of resources, and the presence or absence of predators or potential competitors. As a result of these factors, smaller species may increase in size, often resulting in gigantic island forms (the huge moas of New Zealand, or the giant tortoises of the Galapagos spring to mind). Alternatively, larger species may decrease in size on islands (examples being the dwarf sauropod Magyarosaurus or the multiple instances of dwarf elephants on islands).

Shown below are two such examples from within India’s borders. The Nicobar Islands lie in the Bay of Bengal, and possess a number of endemic bird species. The two examples shown here exemplify these evolutionary patterns on islands demonstrated above. The Nicobar parakeet (Psittacula caniceps) is considerably larger than its mainland relatives, at over 50cm in length (including the very long tail), whereas the Great Nicobar Serpent-Eagle (Spilornis klossi) ranks as one of the smallest birds in the world to bear the title of ‘eagle’, being smaller than a crow, and barely larger than a pigeon! The fauna of the Nicobar Islands remain poorly understood compared to their better-visited northern counterparts the Andamans, and much yet remains to be learned about the ecology of these two bird species. On islands the world over, the very evolutionary diversity that renders them such a fascinating study, also renders their ecosystems particularly vulnerable to the ravages of human-induced change. We have, in a previous post, discussed the extinction of the insular Mamo from the Hawaiian islands, which serves to highlight both the diversity and fragility of island ecosystems. As part of our running theme on the natural history and diversity of birds, this topic is one we will continue to revisit over future posts.

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