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Short-distance Migration in the Dark

Male Little Bustards migrate during the night and use multiple stopover sites.

Jente Ottenburghs
Uppsala University, Sweden

LINKED PAPER
Male post‐breeding movements and stopover habitat selection of an endangered short‐distance migrant, the Little Bustard Tetrax tetrax. Alonso, H., Correia, R. A., Marques, A. T., Palmeirim, J. M., Moreira, F., & Silva, J. P. 2019. IBIS. DOI: 10.1111/ibi.12706. VIEW

Should I stay or should I go now? It is a dilemma that several bird species are challenged with during their lifetime. In some cases, resources are scare after the breeding season and birds are forced to move to other locations. In Portugal and Spain, for example, the Little Bustard (Tetrax tetrax) migrates over short distances when the breeding season is over. To gain more insights into this behaviour, a team of Portuguese researchers used GPS-trackers to map the migration routes of 27 males (Alonso et al. 2019).

Nocturnal Migration
The tracked Little Bustards mainly migrated at night. This strategy has several benefits. First, birds avoid predation by flying nocturnally (Alerstam 2009). Second, the Little Bustards can use the daylight hours to forage and refuel for the next bout of migration. And finally, there are some metabolic benefits to flying at night. In Portugal, temperatures can exceed 35°C during midday. To minimize water loss and prevent overheating, birds reduce their activity during these times of the day (Silva et al. 2015). Migrating at night is then a logical decision.

Whether females follow a similar migration strategy remains to be investigated. The researchers only fitted males with a GPS-tracker. It is, however, likely that females stay at the breeding grounds to raise the young, similar to the Great Bustard (Otis tarda) which lives in the same area (Palacín et al. 2009).

Figure 1 Migration routes of 27 male Little Bustards after breeding at Castro Verde and Vila Fernando SPAs in Portugal. Breeding sites are indicated by white stars and special protected areas with importance for grassland birds are shown in grey.

Stopping on the way
Little Bustards are known to bridge 400-600 kilometres in a single night when crossing the Pyrenees (Villers et al. 2010). Surprisingly, the Portuguese birds made several stopovers on the way to their post-breeding grounds. These birds could easily have covered the distance in a single night. Why did they choose to make stopovers? This decision is probably due to the demanding breeding season. Food resources decline throughout this period and foraging activity is limited by the high summer temperatures. It is likely Little Bustards need to make these stopovers to refuel and rest after an exhausting breeding season.

Members of the Bustard family (Otididae) are extremely selective in their breeding grounds. Little Bustards prefer extensive pastures or agricultural fields for breeding and irrigated fields after the breeding season. During the post-breeding migration, however, Little Bustards seem to be less picky. They stopped at both dry crops and irrigated pastures. The selected areas still offered good protection against predators and ample foraging opportunities. In general, the birds tended to avoid human-made infrastructure. They would keep their distance from roads, but not from power lines. The latter could pose additional threats to the already endangered Little Bustard. It is not easy to see power lines during nocturnal migration which might increase the chances of collision.

Figure 2 Little Bustards made several stopovers during their post-breeding migration. In general, these stopovers were quite short (less than 24 hours).

Conservation Implications
The findings in this study can be used in future management and conservation plans for the Little Bustard. Portuguese populations have declined by about 50 percent, mainly due to habitat loss and degradation. To avoid further decline, open habitat areas should be preserved between the main breeding grounds and the post-breeding areas. This will guarantee the availability of stopover sites and ensure habitat connectivity.

References

Alerstam, T. 2009. Flight by night or day? Optimal daily timing of bird migration. Journal of Theoretical Biology 258: 530–536. VIEW

Palacín, C., Alonso, J.C., Alonso, J.A., Martín, C.A., Magaña, M. & Martin, B. 2009. Differential migration by sex in the Great Bustard: possible consequences of an extreme sexual size dimorphism. Ethology 115: 617–626. VIEW

Silva, J.P., Catry, I., Palmeirim, J.M. & Moreira, F. 2015. Freezing heat: thermally imposed constraints on the daily activity patterns of a free‐ranging grassland bird. Ecosphere 6: 1–13. VIEW

Villers, A., Millon, A., Jiguet, F., Lett, J.M., Attié, C., Morales, M.B. & Bretagnolle, V. 2010. Migration of wild and captive‐bred Little Bustards Tetrax tetrax: releasing birds from Spain threatens attempts to conserve declining French populations. Ibis 152: 254–261. VIEW

About the author

Jente Ottenburghs is the BOU’s Journal Publicity Officer and resident science writer. A postdoc at Uppsala University in Sweden, he is a curious evolutionary biologist with a passion for writing. He obtained his PhD from Wageningen University (the Netherlands) where he studied the genetic consequences of hybridization between several goose species. Currently, he is extending this line of research at Uppsala University. Apart from his goose work, Jente manages the Avian Hybrids Project, a website and blog that gathers the scientific literature on hybridization in birds.

Jente’s personal website
View Jente’s profile on ResearchGate
Follow Jente on Twitter @Jente_O

Image credits

Top right: Little Bustard Tetrax tetrax | Francesco Veronesi | CC BY-SA 2.0 Flickr

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