Scott Susan N, Clegg Sonya M, Blomberg Simon P, Kikkawa Jiro, Owens Ian P F
Department of Zoology and Entomology, University of Queensland, St Lucia, Queensland 4072, Australia.
Evolution. 2003 Sep;57(9):2147-56. doi: 10.1111/j.0014-3820.2003.tb00392.x.
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.
生活在岛屿上的雀形目鸟类，无论在体型还是喙的大小方面，通常都比它们在大陆上的同类更大。对于这种岛屿规则的一种解释是，形态上的变化是一种适应，以促进生态位的扩展。例如，在岛屿雀形目中，喙尺寸的增加可能有助于杂食性觅食，因为这使得它们能够利用更广泛的觅食生态位。在这里，我们利用形态和生态上有差异的绣眼鸟（绣眼鸟科）种群来检验这种解释的三个预测：（1）岛屿种群的觅食生态位比大陆种群更广泛；（2）岛屿栖息种群由个体杂食者组成；（3）在岛屿种群中，体型大小与觅食泛化程度之间存在正相关。我们的结果仅为传统解释提供了部分支持。与核心预测一致，绣眼鸟的岛屿种群确实始终表现出比相对的大陆种群更广泛的觅食生态位。然而，对个体标记鸟类的观察表明，岛屿栖息个体实际上比偶然预期的更具专业性。此外，体型大或喙大本身都与杂食性觅食行为无关。无论我们的测试是基于自然觅食行为还是在实验树上的观察，也无论我们使用来自单个或多个队列的数据，后一个结果都保持一致。综合来看，我们的结果表明，杂食性觅食和生态位扩展并不能完全解释岛屿栖息绣眼鸟的形态变化。因此，我们简要回顾了岛屿种群形态差异的五种替代解释：形态的环境决定、捕食压力降低、生理优化、有限扩散和种内优势。
