S15-4 Large body size in island-dwelling passerines: the roles of insular specialization, niche expansion and ecological release

Birds follow the “island rule”, under which small-bodied forms tend to get larger on islands and large-bodied forms tend to get smaller. The traditional explanation for larger island relatives of small-bodied forms is based on ecological release on islands: islands support relatively few species, interspecific competition thus is weak, selection therefore favors niche expansion and ecological generalism, and ecological generalism is facilitated by larger body size. Anecdotal observations that islanddwelling populations sometimes have unusual feeding habits support this. However, important predictions arising from this hypothesis remain untested, namely that (1) island populations will display a greater range of foraging behaviors than mainland populations, and (2) generalist island populations will be made up of individual generalists rather than a diversity of individual specialists. We tested these predictions using the island-dwelling white-eyes (Zosteropidae) of the Southwest Pacific region, and the Heron Island population of the Capricorn silvereye (Zosterops lateralis chlorocephalus) in particular. Results show that island-dwelling populations of silvereyes are indeed consistently more generalistic than their mainland counterparts when viewed en masse. Contrary to the generalist foraging hypothesis, however, individual island-dwelling silvereyes are actually more specialized than expected by chance alone. Thus generalist foraging and ecological release are not the full explanation for increased body size in these birds.