The effects of supplementary food on the breeding performance of Eurasian reed warblers Acrocephalus scirpaceus; implications for climate change impacts

Understanding the mechanisms by which climate variation can drive population changes requires information linking climate, local conditions, trophic resources, behaviour and demography. Climate change alters the seasonal pattern of emergence and abundance of invertebrate populations, which may have important consequences for the breeding performance and population change of insectivorous birds. In this study, we examine the role of food availability in driving behavioural changes in an insectivorous migratory songbird; the Eurasian reed warbler Acrocephalus scirpaceus. We use a feeding experiment to examine the effect of increased food supply on different components of breeding behaviour and first-brood productivity, over three breeding seasons (2012–2014). Reed warblers respond to food-supplementation by advancing their laying date by up to 5.6 days. Incubation periods are shorter in supplemented groups during the warmest mean spring temperatures. Nestling growth rates are increased in nests provisioned by supplemented parents. In addition, nest predation is reduced, possibly because supplemented adults spend more time at the nest and faster nestling growth reduces the period of vulnerability of eggs and nestlings to predators (and brood parasites). The net effect of these changes is to advance the fledging completion date and to increase the overall productivity of the first brood for supplemented birds. European populations of reed warblers are currently increasing; our results suggest that advancing spring phenology, leading to increased food availability early in the breeding season, could account for this change by facilitating higher productivity. Furthermore, the earlier brood completion potentially allows multiple breeding attempts. This study identifies the likely trophic and behavioural mechanisms by which climate-driven changes in invertebrate phenology and abundance may lead to changes in breeding phenology, nest survival and net reproductive performance of insectivorous birds.

探究气候变异驱动种群动态变化的机制，需要获取联结气候、局地环境、营养资源（trophic resources）、行为与种群统计学（demography）的相关数据。气候变化会改变无脊椎动物种群的出现节律与种群丰度，这可能对食虫鸟类的繁殖表现与种群动态产生显著影响。本研究以食虫性迁徙鸣禽欧柳莺（Eurasian reed warbler, Acrocephalus scirpaceus）为研究对象，探讨食物可获得性对其行为变化的驱动作用。我们在2012至2014年的三个繁殖季内，通过投喂实验探究了食物供给增加对繁殖行为各组分及首窝繁殖生产力的影响。欧柳莺会对食物补给产生响应，其产卵日期可提前至多5.6天。在春季平均温度最高的年份，投喂组的孵化期更短。由投喂亲鸟抚育的巢中，雏鸟生长速率显著提升。此外，巢捕食率有所降低，这可能是由于投喂亲鸟在巢内停留的时间更长，且雏鸟更快的生长速率缩短了卵与雏鸟面临捕食者（及巢寄生者）的脆弱期。上述变化的综合效应为：提前了雏鸟整体离巢的完成日期，并提升了投喂组鸟类首窝繁殖的整体生产力。当前欧柳莺的欧洲种群数量正处于上升态势；本研究结果表明，春季物候提前导致繁殖季早期食物可获得性提升，可通过提高繁殖生产力来解释这一种群增长现象。此外，繁殖季提前完成还可能支持多次繁殖尝试。本研究阐明了气候驱动的无脊椎动物物候与丰度变化，可能通过哪些营养级与行为机制，改变食虫鸟类的繁殖物候、巢存活率与净繁殖性能。