Avian survival on islands

Island species are often predictably different from their mainland counterparts. Milder climates and reduced predation risk on islands have been involved to explain shifts in body size and a suite of life history traits such as clutch size and offspring growth rate. Despite the key role of adult survival on risk taking and reproduction, the prediction that living on islands increases adult survival has yet to be tested systematically. I gathered data on adult annual apparent survival from island and mainland year-round resident species of birds from around the world. With this large dataset (708 species), I found that species of birds living on islands showed higher apparent survival than their mainland counterparts in the two hemispheres and at all latitudes controlling for several known predictors of adult survival including body size, clutch size, and breeding system. These results shed light on the ecological factors that influence survival on islands and extend the life history island syndrome to adult survival.

Methods
I searched the literature for avian studies that reported survival rates. I focused on year-round resident species, as most island species are not migratory. I first consulted earlier extensive reviews of survival rates in birds [15, 18, 20-22]. I complemented this set by performing online database searches using key words such as ‘adult’, ‘survival’, ‘annual’ and ‘birds’. Studies were included in the analysis if they met the following criteria: (1) the study provided estimates of adult annual survival for year-round resident species, (2) survival was not based on age ratios or DNA fingerprinting, (3) the study population was not subject to extensive harvest or artificial conditions that can influence survival such as predator control, (4), the study did not include urban populations or recently introduced species. Most studies reported survival estimates for two sexes combined but if this was not the case I took the arithmetic mean of the two sexes.

岛屿物种通常与其大陆近缘类群存在可预测的差异。岛屿上更温和的气候与更低的捕食风险，曾被用于解释体型变化以及一系列生活史特征的改变，例如窝卵数与后代生长速率。尽管成体存活率对于冒险行为与繁殖具有关键作用，但“栖息于岛屿可提升成体存活率”这一假说仍未得到系统性检验。我收集了全球范围内岛屿与大陆留居鸟类的成体年度表观存活率（apparent survival）数据。依托这份涵盖708个物种的大型数据集，我发现：在两个半球以及所有纬度区间内，在控制了体型、窝卵数与繁殖系统等已知的成体存活率影响因子后，栖息于岛屿的鸟类相较于其大陆近缘类群具有更高的表观存活率。本研究结果阐明了影响岛屿物种存活率的生态因子，并将生活史岛屿综合征（life history island syndrome）的范畴拓展至成体存活率维度。

方法
我检索了报道鸟类存活率的相关文献，研究对象限定为留居型物种，因多数岛屿物种不具有迁徙习性。我首先参考了此前针对鸟类存活率的多篇权威综述文献[15, 18, 20-22]。随后通过关键词检索在线数据库对该文献集进行补充，检索词包括“成体”“存活率”“年度”与“鸟类”。纳入本分析的研究需满足以下标准：（1）研究提供了留居型物种的成体年度存活率估算值；（2）存活率估算未基于年龄比例或DNA指纹技术；（3）研究种群未遭受高强度捕获或可影响存活率的人工干预（例如捕食者管控）；（4）研究未包含城市种群或近期引入的物种。多数研究会同时报道雌雄两性的存活率估算值，若未分别报告，则取两性的算术平均值。