Cooper Nathan W, Dossman Bryant C, Berrigan Lucas E, Brown J Morgan, Brunner Alicia R, Chmura Helen E, Cormier Dominic A, Bégin-Marchand Camille, Rodewald Amanda D, Taylor Philip D, Tonra Christopher M, Tremblay Junior A, Marra Peter P
Migratory Bird Center, Smithsonian's National Zoo and Conservation Biology Institute, 3001 Connecticut Ave. NW - MRC 5503, Washington, DC, 20008, USA.
Department of Biology and McCourt School of Public Policy, Georgetown University, 37th and O Streets NW, Washington, DC, 20057, USA.
Mov Ecol. 2023 May 1;11(1):24. doi: 10.1186/s40462-023-00382-5.
Each spring and fall billions of songbirds depart on nocturnal migrations across the globe. Theory suggests that songbirds should depart on migration shortly after sunset to maximize their potential for nightly flight duration or to time departure with the emergence of celestial cues needed for orientation and navigation. Although captive studies have found that songbirds depart during a narrow window of time after sunset, observational studies have found that wild birds depart later and more asynchronously relative to sunset than predicted.
We used coded radio tags and automated radio-telemetry to estimate the time that nearly 400 individuals from nine songbird species departed their breeding or wintering grounds across North America. We also assessed whether each species was most likely beginning long-distance migratory flights at departure or instead first making non-migratory regional flights. We then explored variation in nocturnal departure time by post-departure movement type, species, age, sex, and season.
We found that 90% of individuals from species that were likely initiating long-distance migratory flights departed within 69 min of civil dusk, regardless of species, season, age, or sex. By contrast, species that likely first made non-migratory regional movements away from the migratory destination departed later and more asynchronously throughout the night. Regardless of post-departure movement type, 98% of individuals departed after civil dusk but otherwise showed no preference in relation to twilight phase.
Although the presence of celestial orientation cues at civil dusk may set a starting point for departure each night, the fact that species likely beginning long-distance migration departed earlier and more synchronously relative to civil dusk than those first making non-migratory regional movements is consistent with the hypothesis that departing promptly after civil dusk functions to maximize the potential for nightly flight duration and distance. By studying the onset of migration, our study provides baseline information about departure decisions that may enhance our understanding of departure timing throughout migration.
每年春秋两季，数十亿只鸣禽会在夜间进行全球范围的迁徙。理论表明，鸣禽应在日落后不久就开始迁徙，以最大化其夜间飞行时长的潜力，或者使其出发时间与定向和导航所需的天体线索出现时间相契合。尽管圈养研究发现鸣禽在日落后的一段狭窄时间内出发，但观察研究发现，野生鸟类相对于日落的出发时间更晚且更不同步，超出了预期。
我们使用编码无线电标签和自动无线电遥测技术，来估计北美九种鸣禽近400只个体离开其繁殖地或越冬地的时间。我们还评估了每个物种在出发时最有可能是开始长途迁徙飞行，还是首先进行非迁徙性的区域飞行。然后，我们通过出发后的移动类型、物种、年龄、性别和季节，来探究夜间出发时间的变化。
我们发现，可能开始长途迁徙飞行的物种中，90%的个体在民用黄昏后的69分钟内出发，无论物种、季节、年龄或性别如何。相比之下，可能首先从迁徙目的地进行非迁徙性区域移动的物种出发时间更晚，且在整个夜间更加不同步。无论出发后的移动类型如何，98%的个体在民用黄昏后出发，但在黄昏阶段没有其他偏好。
尽管民用黄昏时天体定向线索的存在可能为每晚的出发设定一个起点，但与首先进行非迁徙性区域移动的物种相比，可能开始长途迁徙的物种相对于民用黄昏出发更早且更同步，这一事实与民用黄昏后迅速出发能最大化夜间飞行时长和距离潜力的假设相一致。通过研究迁徙的开始，我们的研究提供了有关出发决策的基线信息，这可能会增强我们对整个迁徙过程中出发时间的理解。
