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Jayden Wilson
Jayden Wilson

__LINK__ Download The Winds Of Autumn

The Oriental Stork (Ciconia boyciana) breeds in southeastern Siberia and parts of northeast China, and winters mainly in southeast China. Although the autumn migration pattern of Oriental Storks has been previously described, differences between spring and autumn migration travel speed in relation to wind assistance were unknown.

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For long distance migratory soaring birds (such as storks), relative duration of spring and autumn migration likely relates to the interaction between imperative for earliest arrival to breeding grounds and seasonal meteorological conditions experienced en route.

That weather conditions affect the timing and flight behaviour of long-distance bird migration is well known (Shamoun-Baranes et al. 2017; Becciu et al. 2019). For instance, the migration of large terrestrial soaring birds may be affected by local atmospheric conditions, as it is known that they prefer to use thermals to gain altitude before they glide towards their destination (Norberg 2012). However, of all atmospheric conditions, wind has perhaps the greatest effect on migration of birds (Liechti 2006; Mellone et al. 2012; Safi et al. 2013; Rotics et al. 2016; Vidal-Mateo et al. 2016). For example, Becciu et al. (2018) found that soaring birds exploit tailwinds to move faster, confirmed by Shamoun-Baranes et al. (2003) who showed the migration ground speeds of White Storks (Ciconia ciconia) increased with tailwinds in both spring and autumn, but that this decreased in headwinds. Moreover, migration intensity also increases in tailwind conditions (Erni et al. 2002; Fox et al. 2003; Becciu et al. 2019), even within seasons at the same site in response to different wind directions (e.g. Desholm et al. 2014).

Several studies have attempted to analyse relationships between weather factors at fixed locations (such as meteorological stations) and the migration of large soaring birds. Although not all studies have revealed an impact of wind on migration (for example, wind had no effect on Osprey Pandion haliaetus migration; Thorup et al. 2006), many studies show selection for prevailing winds by large birds on, or about to undertake migration (Allen et al. 1996; Spaar and Bruderer 1996; Meyer et al. 2000).

As large, heavy soaring birds, storks are known to exploit tailwinds to aid their long distant movement (Becciu et al. 2018). Although the linkages between wind and migration are well studied for some species (Shamoun-Baranes et al. 2003; Vansteelant et al. 2015; Rus et al. 2017), no study has compared contrasting seasonal wind assistance on spring versus autumn migration of Oriental Storks (Ciconia boyciana), which is listed as an IUCN Endangered species (IUCN 2018). Despite a rich literature relating to the relative speed of spring versus autumn migrations (e.g. Alerstam and Hedenstrom 1998; Nilsson et al. 2013), there remain relatively few studies of the relative contributions of seasonal wind assistance to the speed and duration (i.e. the time taken to migrate between the breeding and wintering grounds) of spring and autumn migrations (e.g. Koelzsch et al. 2016). To address the knowledge gap for the Oriental Stork we therefore here present a study comparing the differential effects of wind parameters on a soaring bird during spring and autumn migration episodes.

To test for significant differences between autumn and spring migration in each of the migration parameters (except for arrival and departure dates), we used paired t-tests (for those meeting the assumptions of normality and homogeneity of variance) and Wilcoxon signed-rank tests for the remaining parameters. We also used independent t-tests for differences in tailwinds in autumn versus spring, and Wilcoxon signed-rank tests for differences in daily flying speed during tail and head winds. We used a generalized additive model (GAM) to test for a relationship between daily flying speed and wind, as well as to test whether the difference in tailwind (calculated by subtracting the tailwinds in autumn from the tailwinds in spring) could explain the difference in daily flying speed (using the same calculation as for winds) between spring and autumn migration. All modelling and statistical analysis were performed using R software (R Development Core Team 2017).

Individual autumn migration routes and stopover sites of 18 Oriental Storks (Ciconia boyciana) derived from GPS/GSM telemetry devices. The distribution of breeding area and non-breeding area from BirdLife International

GAM plot from the best fitting function of the difference in seasonal tailwind (X-axis) on the difference in seasonal average daily flying speed (Y-axis). The scale of the Y-axis reflects the relative importance of covariate in the model. Dashed lines represent two standard error boundaries around the covariate. Vertical lines along the X-axis represent a rugplot of data points used in the analysis to show the distribution of data points over the range of the difference in seasonal tailwinds

The benefits of migrating with tailwinds have been widely reported in other large bird species, such as Honey Buzzards (Pernis apivorus), White Storks and Ospreys (Shamoun-Baranes et al. 2003; Vansteelant et al. 2015; Rus et al. 2017). However, direct comparisons can be difficult, because for day migrating species that make use of thermals, day length greatly affects migratory duration and speed (Mellone et al. 2012). However, our study is the first to show that the speed of migration in the Oriental Stork is seasonally affected by differences in the benefit gained from tailwinds throughout the migration episodes, which predictably differed between spring and autumn. It is of course important to stress that our findings are from young Oriental Storks (which do not breed in their first year), and the behaviour of adults may be different from first year birds. In other species, age specific differences in migration schedules have been demonstrated, usually becoming more similar to adult migration patterns with age (Hake et al. 2003; Mueller et al. 2013; Sergio et al. 2014). Generally, in large-bodied birds, males arrive earlier than females, which may reflect their need to secure a breeding territory (Rotics et al. 2018), but in our study we could not distinguish between the sexes and territorial defence was not relevant.

Numerous studies of soaring birds have shown relationships between strength of tailwinds and speed of migratory birds. For example, peak movements or departures of migrants are associated with periods of favourable tailwinds (Allen et al. 1996; Spaar and Bruderer 1996; Meyer et al. 2000), while cross-country speeds of Steppe Eagles (Aquila nipalensis) increased with the increasing tailwind in southern Israel (Spaar and Bruderer 1996). Hence, wind direction and speed have the potential to substantially affect daily travel speed (Vansteelant et al. 2015).

Oriental Storks are c. 40% heavier than White Storks (Dunning 2007), although structurally and ecologically they are very similar species. Hence, it seems highly likely that the reversal in relative duration of spring and autumn migration in these two species is linked to the degree and strength of tailwinds to which they are exposed during migration along their respective flyway corridors.

We therefore contend that comparing the duration of spring versus autumn migration in relation to theories about the imperative to arrive first to breeding areas to secure nesting resources is something of a straw man. There is no doubt that both Oriental and White Storks have a similar imperative to arrive as early as possible to occupy and defend valuable nest sites, yet Oriental Storks show no significant difference between autumn and spring migration duration, while White Storks take longer to complete spring migration than covering the same distance in autumn. This study and that of Shamoun-Baranes et al. (2003) show both species are dependent upon tailwind assistance to increase the speed of migration and both studies strongly imply that the seasonal headwind/tailwind ratio in spring versus autumn in Oriental and White Storks contributes to the duration of the migration episode.

We believe that the seasonal differences in the travel speed of Oriental Storks during migration are related to the physical environmental conditions they encounter, which differ in spring compared to autumn. Their faster rate of travel on spring migration can be explained by stronger tailwinds during that season, yet there was no significant difference in the overall duration of autumn and spring migration. The reason might be because the actual travel duration constitutes a relatively small part of overall migration duration, but the stopover duration (which contributes most to migration duration) showed much larger individual variation in spring than autumn. This seems to contrast with the White Storks, which took far longer to complete spring migration than when covering the same distance in autumn, potentially because of more favourable tailwinds in that season compared with headwinds that are encountered in spring (Shamoun-Baranes et al. 2003). This confirms that long distance soaring migrants may strive to shorten the duration of spring migration to ensure earliest arrival at breeding grounds to ensure territorial defence. However, storks are inevitably forced to migrate under the prevailing meteorological conditions in spring, which may differ radically between different flyway populations, dependent on the peculiar local weather patterns to which they are exposed. This finding also confirms the vulnerability of such populations to changes in prevailing atmospheric conditions under current climate change predictions if the prevailing seasonal wind patterns begin to change in the near future, as predicted that they will. Research indicates that changes in wind caused by global warming may have major impacts in the future (McInnes et al. 2011) which could potentially affect migrating storks and other large-bodied avian species.


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