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Adaptive Evolution and Shifts in Niche Occupation in Island Birds

Allen Keast
Biotropica
Vol. 2, No. 2 (Dec., 1970), pp. 61-75
DOI: 10.2307/2989764
Stable URL: http://www.jstor.org/stable/2989764
Page Count: 15
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Adaptive Evolution and Shifts in Niche Occupation in Island Birds
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Abstract

Adaptive evolution in island birds is investigated with special reference to the birds of Tasmania, a continental island 26.000 square miles in area that lies 140 miles off southern Australia. The avifauna is a typical insular one being numerically impoverished (only 43 species of passerine birds compared with 89 in equivalent habitats on the adjacent mainland), and lacking certain "basic kinds" of birds (e.g., true trunk feeders are absent) The study emphasizes shifts in vertical feeding zones and in morphological attributes associated with perching and feeding (e.g., bill, tarsus, hallux). The findings are as follows: 1. A series of island species have moved into the vacant trunk feeding and underexploited arboreal, foliage-gleaning, adaptive zones; 2. these are species that, on the adjacent Australian mainland, already feed to some slight extent in these zones; 3. the shifts invariably involve a broadening of, or increased diversity in, feeding; 4. there is a broad redivision of ecological roles and adaptive niches on the island; vacant niches are eliminated, and a new state of integration and balance is achieved within the avifauna.

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