Autumn migration of the migrant hawker (Aeshna mixta) at the Baltic coast

Yvonne Oelmann; Diana Fiedler; Rune Michaelis; Meelis Leivits; Andreas Braun; Philipp Gschwind; Harald Neidhardt; Christoph Willigalla

doi:10.1186/s40462-023-00415-z

Autumn migration of the migrant hawker (Aeshna mixta) at the Baltic coast

Mov Ecol. 2023 Aug 24;11(1):52. doi: 10.1186/s40462-023-00415-z.

Authors

Yvonne Oelmann¹, Diana Fiedler², Rune Michaelis^{2

3}, Meelis Leivits⁴, Andreas Braun⁵, Philipp Gschwind^{2

6}, Harald Neidhardt², Christoph Willigalla⁷

Affiliations

¹ Geoecology, Department of Geosciences, University of Tübingen, 72070, Tübingen, Germany. yvonne.oelmann@uni-tuebingen.de.
² Geoecology, Department of Geosciences, University of Tübingen, 72070, Tübingen, Germany.
³ Lower Saxon Wadden Sea National Park Authority, 26382, Wilhelmshaven, Germany.
⁴ Estonian Environment Agency, Nigula Nature Centre, 86107, Reinu village, Estonia.
⁵ Geoinformatics, Department of Geosciences, University of Tübingen, 72070, Tübingen, Germany.
⁶ GÖG - Gruppe für ökologische Gutachten, 70599, Stuttgart, Germany.
⁷ Willigalla Ökologische Gutachten, 55124, Mainz, Germany.

Abstract

Background: Migratory insects are important for the provision of ecosystem services both at the origin and destination sites but - apart from some iconic species - the migration routes of many insect species have not been assessed. Coastlines serve as a funnel where migrating animals including insects accumulate. Migratory behaviour and captures of dragonflies in bird traps suggest autumn migration of dragonflies along coastlines while the origin and regularity of this migration remain unclear.

Methods: Dragonfly species were caught at the bird observatory Kabli at the Baltic coast in Estonia in 2009, 2010 and 2015. For the 2015 data set, we used a stable hydrogen (H) approach to trace the potential natal origin of the migrant hawker (Aeshna mixta).

Results: 1079 (2009), 701 (2010) and 88 (2015) A. mixta individuals were caught during the study periods (35, 37 and 11 days in 2009, 2010 and 2015, respectively). The migration period lasted from end of August to end of September. Based on the results from our stable isotope analysis, we identified two populations of A. mixta: One (range of isotope signatures of non-exchangeable H [δ²H_{n wing}]: -78‰ to -112‰) had a local likely origin while the other (δ²H_{n wing}: -113‰ to -147‰) migrated from northerly directions even in headwind from the South. The former showed an even sex ratio whereas the actively migrating population was dominated by males.

Conclusions: Our results suggest a regular southbound autumn migration of A. mixta along the Baltic coast. However, nearly half of the sampled individuals originated from the surroundings suggesting either no, partial or "leap-frog" migration. Contrary to our expectation, A. mixta did not select favourable wind conditions but continued the southbound autumn migration in the flight boundary layer even in case of headwinds. The dominance of males might indicate migration as a result of competition for resources. Further repeated, large-scale studies along the Baltic coast are necessary to pinpoint the migratory pattern and the reason for migration of A. mixta. Such studies should also comprise locations north of the known species range of A. mixta because of the rapid climate-change induced range expansion.

Keywords: Baltic coast; Bird observatory; Dragonfly; Hydrogen; Isoscape; Migration; Stable isotopes.