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1 March 2014 A hard nut to crack: rapid evolution in the Kona Grosbeak of Hawaii for a locally abundant food source (Drepanidini: Chloridops kona)
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Abstract

The Kona Grosbeak (Chloridops kona), last seen in 1892, was restricted to a small mid-elevation area on the leeward (Kona) side of the island of Hawaii. It was reported to feed almost exclusively on the fruits of the naio tree (Myoporum sandwicense), which are extremely hard, requiring a force of perhaps 400 newtons or more to open. The morphology of the skull and mandible of the Kona Grosbeak was accordingly much more modified for the hypertrophied adductor musculature needed to crack naio fruits than its closest relative, the Wahi Grosbeak (Chloridops wahi), which has been found as fossils on most of the other main Hawaiian islands. The Wahi Grosbeak is here hypothesized to have fed on less resistant seeds, such as those of the Hawaiian prickly-ash (Zanthoxylum spp.). Naio is widely distributed in diverse habitats in the Hawaiian Islands but its fruits were apparently an energetically viable food source only on relatively young (ca. 1200 year old) aa lava flows on the dry side of Hawaii, where the plants produce abundant flowers and fruit on a year-round basis. The Kona Grosbeak therefore appears to have evolved in less than the ca. 400,000 years since Mauna Loa became high enough to create a rain shadow and develop younger aa flows and associated vegetation.

© Copyright 2014 by the Wilson Ornithological Society
Storrs L. Olson "A hard nut to crack: rapid evolution in the Kona Grosbeak of Hawaii for a locally abundant food source (Drepanidini: Chloridops kona)," The Wilson Journal of Ornithology 126(1), 1-8, (1 March 2014). https://doi.org/10.1676/13-046.1
Received: 26 March 2013; Accepted: 1 June 2013; Published: 1 March 2014
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