The Yellow-browed Warbler (Phylloscopus inornatus) as a model to understand the mechanisms of vagrancy and its potential significance for the evolution of new migratory routes

Yellow-browed Warbler, by Moss Taylor / BTO

Author(s): Dufour, P., Åkesson, S., Hellström, M., Hewson, C., Lagerveld, S., Mitchell, L., Chernetsov, N., Schmaljohann, H. & Crochet, P-A.

Published: December 2022

Journal: Movement Ecology

Digital Identifier No. (DOI): 10.1186/s40462-022-00345-2

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Why and how new migration routes emerge remain fundamental questions in ecology, particularly when viewed in the context of the current climate and biodiversity crises. We have already seen changes to migratory behaviour in some species, both in terms of the timing of movements and routes used, but identifying such changes at their earliest stage can be especially challenging, as this paper reveals.

In addition to their regular movements, birds are also known for their propensity to move beyond their geographic range limits, a phenomenon known as vagrancy, and a behaviour that provides birdwatchers with an opportunity to encounter species new to their local area. The earliest stages in the evolution of new migration routes can easily be confused with vagrancy – both involve small numbers of individuals encountered outside of their regular breeding, non-breeding or migratory ranges. If sufficient individuals survive the rigours of these journeys, return to their breeding grounds and pass on their new migration route to their offspring then a new migration route can evolve.

This paper uses the Yellow-browed Warbler as a case study to explore the challenges of distinguishing regular migration from vagrancy. This Siberian species normally winters in southern Asia but has increased in occurrence in Western Europe over recent years. The researchers review the available evidence, suggesting that the Yellow-browed Warbler is still mostly a vagrant in Western Europe, but might be establishing a new migration route initiated by vagrants. Importantly, the paper then outlines possible approaches to establish whether some individuals really undertake regular migratory movements between Western Europe and Siberia. By doing this, the researchers lay the foundations for future work centred on this enigmatic species.

Abstract

Why and how new migration routes emerge remain fundamental questions in ecology, particularly in the context of current global changes. In its early stages, when few individuals are involved, the evolution of new migration routes can be easily confused with vagrancy, i.e. the occurrence of individuals outside their regular breeding, non-breeding or migratory distribution ranges. Yet, vagrancy can in theory generate new migration routes if vagrants survive, return to their breeding grounds and transfer their new migration route to their offspring, thus increasing a new migratory phenotype in the population. Here, we review the conceptual framework and empirical challenges of distinguishing regular migration from vagrancy in small obligate migratory passerines and explain how this can inform our understanding of migration evolution. For this purpose, we use the Yellow-browed Warbler (Phylloscopus inornatus) as a case study. This Siberian species normally winters in southern Asia and its recent increase in occurrence in Western Europe has become a prominent evolutionary puzzle. We first review and discuss available evidence suggesting that the species is still mostly a vagrant in Western Europe but might be establishing a new migration route initiated by vagrants. We then list possible empirical approaches to check if some individuals really undertake regular migratory movements between Western Europe and Siberia, which would make this species an ideal model for studying the links between vagrancy and the emergence of new migratory routes.

Notes

Authors of this study warmly thank: Fair Isle Bird Observatory, the EURING organisation, Hisko de Vries (https://waarneming.nl) and Jochen Dierschke for the data used to illustrate this paper; all the European volunteer observers and ringers who have contributed to compiling these occurrence and ringing data. Authors express their gratitude to Christophe de Franceschi, Bo Petersson, Anatoly Shapoval, Stephan Tillo and Maxime Zucca.

This work has been supported by a PhD grant to P.D. awarded by the Labex OSUG@2020 (Investissements d’avenir – ANR10LABX56). Funding was granted from the Deutsche Forschungsgemeinschaft (DFG) within the Sonderforschungsbereich (SFB) 1372 “Magnetoreception and Navigation in Vertebrates” (INST 184/203–1, project number 395940726) to H.S. S.Å. was supported by project grants from the Swedish Research Council (2016–05342) and the Carl Trygger Foundation (CTS: 17518), and a Linnaeus grant (349–2007-8690) from the Swedish Research Council and Lund University to the Centre for Animal Movement Research (CAnMove) at Lund University. This is contribution no 323 from Ottenby Bird Observatory.

Staff Author(s)

Chris Hewson

Senior Research Ecologist

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