Abstract
For many migratory species, migration can represent a significant part of the annual cycle and the strategies used to move between the breeding and non-breeding areas vary considerably. Weather conditions are important during migration, particularly wind and temperature, and can play a crucial role in the timing of events during the annual cycle of migratory birds. When timing of specific events is important, for example spring arrival and laying dates, the effects of weather on the previous migration might lead to important fitness consequences, as it may alter migration speed. During spring, Icelandic whimbrels Numenius phaeopus islandicus display two main migratory behaviours: a direct flight from the wintering to the breeding sites (direct migration), or, more commonly, two flights with one stopover (stopover migration). We investigated how wind conditions, temperature and spring departure date may drive individuals to adopt either migratory behaviour. Interestingly, we found no differences in wind support during migratory flights, in temperature closer to Iceland or on crosswinds experienced in the region before reaching the main stopover areas. However, when individuals undertook a direct flight, departure date from the wintering sites was on average later, but this was not explained by wind patterns over a period of seven days prior to departure. In addition, we explored the variation at the individual level for three birds that changed migratory behaviour between years. The differences in all variables for these individuals reflected the variation observed at the population level. Overall, in such long migrations, it seems advantageous to perform a shorter flight to a stopover area, from where the weather conditions in the breeding areas may be assessed and avoid the risk of facing stochastic inclement weather prior to breeding, while synchronising time of arrival with conspecifics. In contrast, direct flights seem more common when individuals are time pressed.
Highlights
Migration can represent a significant part of an individual annual cycle and the strategies used to move between the breeding and non-breeding areas vary considerably (Newton, 2007; Hansson and Åkesson, 2014)
The zonal wind component when crossing the region from latitude 37◦N to 50◦N was not different between migratory behaviors at any air pressure level considered (Table 1B and Figures 1B, 2D), and was predominantly westward
We investigated how wind conditions, temperature, and spring departure date may drive individuals to adopt each behavior, and found no differences in wind support during migration, on temperature closer to Iceland or on crosswinds experienced in the region before reaching the main stopover sites
Summary
Migration can represent a significant part of an individual annual cycle and the strategies used to move between the breeding and non-breeding areas vary considerably (Newton, 2007; Hansson and Åkesson, 2014). Wind can be favorable and cause birds to extend the length of migration (Gill et al, 2014), drift due to crosswinds (Grönroos et al, 2013; Horton et al, 2016) or even force stops (Shamoun-Baranes et al, 2010; Klaassen et al, 2017); the effect of temperature on flight has been studied to less extent, but it has been shown to influence individuals’ flight altitude (Senner et al, 2018). Weather can play an important role on the annual cycle of migratory birds and changes in the atmospheric patterns at a large scale may influence whole populations or species, through costs of energy and time during migration (La Sorte and Fink, 2017; La Sorte et al, 2018). Understanding the mechanisms shaping migratory behavior can help predicting how these species will respond to variation in weather patterns
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