Migration strategies and annual space-use in an Afro-Palaearctic aerial insectivore – the European nightjar Caprimulgus europaeus

Obligate insectivorous birds breeding in high latitudes travel thousands of kilometres during annual movements to track the local seasonal peaks of food abundance in a continuously fluctuating resource landscape. Avian migrants use an array of strategies when conducting these movements depending on e.g. morphology, life history traits and environmental factors encountered en route. Here we used geolocators to derive data on the annual space-use, temporal pattern and migratory strategies in an Afro-Palaearctic aerial insectivorous bird species – the European nightjar Caprimulgus europaeus. More specifically, we aimed to test a set of hypothesises pertaining to the migration of a population of nightjars breeding in south-eastern Sweden. We found that the birds wintered across the central and western parts of the southern tropical Africa almost entirely outside the currently described wintering range of the species. The nightjars performed a narrow loop migration across Sahara, with spring Sahel stopovers significantly to the west of autumn stops indicative to an adaptive response to winds during migration. To our surprise, the migration speed was faster in the autumn (119 km d− 1) than in the spring (99 km d− 1), possibly due to the prevailing wind regimes over the Sahara. The estimated flight fraction in both autumn (14%) and spring (12%) was almost exactly as the theoretically predicted 1:7 time relationship between flights and stopovers for small birds. The temporal patterns within the annual cycle indicate that individuals follow alternative spatiotemporal schedules that converge towards the breeding season. The positive relationship between the spatially and temporally distant winter departure and breeding arrival suggests that individuals´ temporal fine-tuning to breeding may be constrained, leading to potential negative fitness consequences.

繁殖于高纬度地区的专性食虫鸟类，会在年度迁徙活动中跋涉数千公里，以追踪持续动态变化的资源景观中当地食物丰度的季节性峰值。候鸟在开展此类迁徙时会采用一系列策略，这取决于其形态、生活史特征以及途中遭遇的环境因素等。本研究使用地理定位器（geolocator）获取了非洲-古北界（Afro-Palaearctic）空中食虫鸟类——欧洲夜鹰（Caprimulgus europaeus）的年度空间利用模式、时间节律与迁徙策略相关数据。更具体而言，本研究旨在检验针对瑞典东南部繁殖种群的欧夜鹰迁徙行为的一系列假说。研究发现，该种群的越冬地位于南热带非洲的中部与西部，几乎完全超出了当前已记录的该物种的越冬分布范围。欧夜鹰开展横跨撒哈拉沙漠的窄环路迁徙，其春季在萨赫勒地区的停歇位点显著位于秋季停歇位点的西侧，这反映了其在迁徙过程中对风力的适应性应对。令人意外的是，秋季迁徙速度（119千米/日）快于春季（99千米/日），这可能与撒哈拉地区的盛行风场有关。研究估算的秋季（14%）与春季（12%）的飞行时长占比，几乎完全契合小型鸟类飞行与停歇时长间理论预测的1:7比例关系。年度周期内的时间节律模式表明，个体采用不同的时空迁徙计划，但最终都会汇聚至繁殖季。越冬出发时间与繁殖抵达时间在空间和时间上均存在显著正相关，这表明个体对繁殖的时间微调可能受到限制，进而可能对适合度产生潜在负面影响。