Morphological Shifts in Island‐dwelling Birds: The Roles of Generalist Foraging and Niche Expansion

Abstract

Passerine birds living on islands are usually larger than their mainland counterparts, in terms of both body size and bill size. One explanation for this island rule is that shifts in morphology are an adaptation to facilitate ecological niche expansion. In insular passerines, for instance, increased bill size may facilitate generalist foraging because it allows access to a broader range of feeding niches. Here we use morphologically and ecologically divergent races of white-eyes (Zosteropidae) to test three predictions of this explanation: (1) island populations show a wider feeding niche than mainland populations; (2) island-dwelling populations are made up of individual generalists; and (3) within insular populations there is a positive association between size and degree of foraging generalism. Our results provide only partial support for the traditional explanation. In agreement with the core prediction, island populations of white-eye do consistently display a wider feeding niche than comparative mainland populations. However, observations of individually marked birds reveal that island-dwelling individuals are actually more specialized than expected by chance. Additionally, neither large body size nor large bill size are associated with generalist foraging behavior per se. These latter results remained consistent whether we base our tests on natural foraging behavior or on observations at an experimental tree, and whether we use data from single or multiple cohorts. Taken together, our results suggest that generalist foraging and niche expansion are not the full explanation for morphological shifts in island-dwelling white-eyes. Hence, we review briefly five alternative explanations for morphological divergence in insular populations: environmental determination of morphology, reduced predation pressure, physiological optimization, limited dispersal, and intraspecific dominance.

Contrasting Levels of Variation in Neutral and Quantitative Genetic Loci on Island Populations of Moor Frogs (Rana arvalis)

“…Our results conform to the general pattern of lowered marker gene diversity (Frankham 1997) and an increased degree of population differentiation (Johnson et al 2000;Scott et al 2003) in island as compared to mainland populations. In particular, moor frogs on the island of Gotland seem to have lost their genetic variability in neutral marker genes since the island was colonized, but this loss of genetic diversity is not discernible in the levels of additive genetic variation in ecologically important traits.…”

Divergence in New Holland Honeyeaters (Phylidonyris novaehollandiae): evidence from morphology and feeding behavior

“…The increase in resource use in island birds matches a trend observed for generalist foragers (Christensen and ). It is unclear if our observations of a wider niche breadth in island birds are the result of many individual foraging specialists or a population of generalist foragers, as is often assumed (see Werner and Sherry 1987;Bolnick et al 2003;Scott et al 2003). Irrespective of the underlying structure of individual specialist or generalists, a wider niche breadth would be favored by the absence of some bird species on the island.…”

Molecular and Phenotypic Data Support the Recognition of the Wakatobi Flowerpecker (Dicaeum kuehni) from the Unique and Understudied Sulawesi Region

“…Furthermore, this morphological divergence is suggestive of major ecological differences between these populations. The increases in tarsus length and bill size (length and depth) seen in Wakatobi populations are associated with changes in foraging and feeding ecology, respectively, both of which are suggestive of niche expansion [46], [47]. The observed increases in body size (which can be indexed from wing length and weight) may be due to reduced interspecific competition, increased intraspecific competition (with comparatively higher population densities) and/or variation in energetic constraints and physiological optimisation [46]–[49].…”