Yellow-browed Warbler ringing project launches

A new Yellow-browed Warbler colour-ringing project has launched across Britain and Ireland – and birders are being asked to report any sightings.

Colour rings have been distributed to ringers across Britain and Ireland, initially focusing on bird observatories and other key migration sites from the Northern Isles down to Cornwall.

Yellow-browed Warbler is an ostensibly Asian passerine, breeding in Siberian taiga forests and mostly wintering in South-East Asia. The last few decades have seen a substantial rise in the number of birds recorded in Western Europe. This has led to Yellow-browed Warbler being considered an expected autumn migrant, rather than a rare vagrant, in Britain. 

Birders are being asked to keep an eye out for colour-ringed Yellow-browed Warblers this autumn. This bird on Fair Isle was the first to be ringed in the project (Alex Penn).

Change in status

There are a diverse range of hypotheses to explain this change in status, not all of which are mutually exclusive (Dufour et al 2022). For example, the increase may simply reflect an increase in vagrancy associated with population growth and/or range expansion.

Alternatively, the change in status may reflect a change in the species' migration ecology and the establishment of a nascent wintering population far from the species' regular range.

This later hypothesis referred to as 'pseudo vagrancy' (Gilroy and Lees, 2003) has recently been established to have occurred in Richard's Pipit (Lees and Gilroy, 2021; Dufour et al 2021).

The Yellow-browed Warbler project

This project, with the use of colour-ringing, aims to increase the ring recovery rate of Yellow-browed Warbler in order to learn more about the movements of the species within Western Europe. More specifically, it aims to look at the trajectory of movements and investigate whether there is a subsequent southbound movement after birds initially follow a western trajectory reaching the westernmost limits of Europe in autumn.

Additionally, the project aims to identify migration stop-over duration and wintering origin of Yellow-browed Warblers arriving in Britain and Ireland, with colour rings increasing the likelihood of identifying winter site fidelity.

The scheme will use a combination of three very small colour rings (each weighing 0.0081 g), one above the metal ring on the right tarsus and two on the left tarsus. Initially seven different colours will be used (red, orange, blue, green, yellow, purple and white). 

When out birding this autumn, please check any Yellow-browed Warblers for colour rings and report all sightings to Dan Gornall ([email protected]).

References

Dufour, P, et al. 2022. The Yellow-browed Warbler (Phylloscopus inornatus) as a model to understand vagrancy and its potential for the evolution of new migration routes. Mov Ecol 10, 59. DOI: doi.org/10.1186/s40462-022-00345-2
Dufour, P, et al. 2021. A new westward migration route in an Asian passerine bird. Current Biology, 31.
Gilroy, J, and Lees, A C. 2003. Vagrancy theories: Are autumn vagrants really reverse migrants? British Birds 96(9)
Gilroy, J, and Lees, A C. 2021. Bird migration: When vagrants become pioneers. Current Biology 31 (24). DOI: doi.org/10.1016/j.cub.2021.10.058