Alerstam Thomas
Department of Animal Ecology, Lund University, Ecology Building, SE-22362 Lund, Sweden.
J Theor Biol. 2009 Jun 21;258(4):530-6. doi: 10.1016/j.jtbi.2009.01.020.
Many migratory bird species fly mainly during the night (nocturnal migrants), others during daytime (diurnal migrants) and still others during both night and day. Need to forage during the day, atmospheric structure, predator avoidance and orientation conditions have been proposed as explanations for the widespread occurrence of nocturnal migration. However, the general principles that determine the basic nocturnal-diurnal variation in flight habits are poorly known. In the present study optimal timing of migratory flights, giving the minimum total duration of the migratory journey, is evaluated in a schematic way in relation to ecological conditions for energy gain in foraging and for energy costs in flight. There exists a strong and fundamental advantage of flying by night because foraging time is maximized and energy deposition can take place on days immediately after and prior to the nocturnal flights. The increase in migration speed by nocturnal compared with diurnal migration will be largest for birds with low flight costs and high energy deposition rates. Diurnal migration will be optimal if it is associated with efficient energy gain immediately after a migratory flight because suitable stopover/foraging places have been located during the flight or if energy losses during flight are substantially reduced by thermal soaring and/or by fly-and-forage migration. A strategy of combined diurnal and nocturnal migration may be optimal when birds migrate across regions with relatively poor conditions for energy deposition (not only severe but also soft barriers). Predictions about variable timing of migratory flights depending on changing foraging and environmental conditions along the migration route may be tested for individual birds by analysing satellite tracking results with respect to daily travel routines in different regions. Documenting and understanding the adaptive variability in daily travel schedules among migrating animals constitute a fascinating challenge for future research.
许多候鸟主要在夜间飞行（夜行性候鸟），其他的在白天飞行（日行性候鸟），还有一些则在白天和夜间都飞行。白天觅食的需求、大气结构、躲避捕食者以及定向条件都被认为是夜行性迁徙广泛存在的原因。然而，决定飞行习性基本昼夜变化的一般原则却鲜为人知。在本研究中，从觅食能量获取和飞行能量消耗的生态条件出发，以示意性的方式评估了能使迁徙旅程总时长最短的迁徙飞行最佳时间。夜间飞行存在一个强大且根本的优势，因为觅食时间得以最大化，并且能量储存可以在夜间飞行前后的日子里进行。对于飞行成本低且能量储存率高的鸟类来说，与日行性迁徙相比，夜行性迁徙速度的提升最为显著。如果日行性迁徙与迁徙飞行后立即高效获取能量相关，比如在飞行过程中找到了合适的中途停留/觅食地点，或者通过热气流翱翔和/或边飞边觅食迁徙大幅降低了飞行中的能量损失，那么日行性迁徙将是最佳选择。当鸟类穿越能量储存条件相对较差的区域（不仅是严峻的障碍，还有温和的障碍）时，昼夜结合的迁徙策略可能是最佳的。通过分析卫星追踪结果中不同区域的每日行程安排，可以针对个体鸟类检验关于迁徙飞行时间变化取决于沿迁徙路线不断变化的觅食和环境条件的预测。记录和理解迁徙动物每日行程安排中的适应性变化，是未来研究面临的一个引人入胜的挑战。
