Plumage variation in autumn Willow Warblers

Mark James Pearson looks at the┬áplumage variation exhibited by Willow Warblers during last September’s major fall of continental passerines, with additional examples of equally striking late autumn individuals from the last couple of years.

Willow Warbler, Carr Naze, 16th September 2014

Willow Warbler, Carr Naze, 16th September 2014. A striking bird that arrived in mist and drizzle and frequented clifftop vegetation (Mark Pearson)

Within the species list of the about-to-be-published 2014 Filey Report, mention is made of the variation in plumage of Willow Warblers Phylloscopus trochilus that we encountered during the memorable influx of migrants in mid-September of last year (and refers readers to the photos published here). The productive easterly airflow which brought them to Filey also delivered a significant arrival of continental passerines, which included a host of rarer visitors – both Little and Rustic Buntings, two candidate Siberian Lesser Whitethroats, a Wood Warbler, six Yellow-browed Warblers and three Red-breasted Flycatchers were among the bounty, providing a strong indication of the geographical source of these distinctly un-British-looking new arrivals.

Willow Warbler, Carr Naze, 16th September 2014

Willow Warbler, Carr Naze, 16th September 2014. The same bird as pictured above. This particular bird caused plenty of confusion in the fog, and was very likely the source of reports of much rarer warblers in this spot during the day (Mark Pearson)

From where exactly? It’d be good to know…. racial attribution and taxonomic consensus are notoriously tricky and controversial, with much still to be discovered on the subject and little in the way of set criteria for definitive field identification of races or forms; however, it seems more than likely that many of the birds we received during this period (and during similar arrivals in recent years) were from northern and/or eastern populations.

Willow Warbler, Carr Naze, 16th September 2014

Willow Warbler, Carr Naze, 16th September 2014. Another striking bird, equally ‘cold’ but substantially paler (and with paler legs) (Mark Pearson)

Willow Warbler, Carr Naze, 16th September 2014

Willow Warbler, Carr Naze, 16th September 2014. One of a very few ‘yellow’ Willow Warblers in the area during the period (and much less of a headache!) (Mark Pearson)

Willow Warbler, Arndale, 18th October 2013. This bird arrived the day before, in the same hour as Dusky Warbler, Siberian Chiffchaff, three Yellow-browed Warblers and a Siberian Lesser Whitethroat. (Mark Pearson)

Willow Warbler, Arndale, 18th October 2013. This bird arrived the day before, in the same hour as Dusky Warbler, Siberian Chiffchaff, three Yellow-browed Warblers and a Siberian Lesser Whitethroat (Mark Pearson)

Then there are those birds appear even later in the season – in mid to late October – which can be even more striking. The birds pictured above and below (from 2013 and 2014 respectively) were clearly from a long way away, and are arguably just as interesting as the Siberian and Asian vagrants they arrived with…

Willow Warbler, 30th October 2014. Photographed as it dropped onto the very tip of Carr Naze during a huge fall of late autumn passerines, this bird exhibited almost Bonelli's Warbler-like tones. (Mark Pearson)

Willow Warbler, 30th October 2014. Photographed as it dropped onto the very tip of Carr Naze during a huge fall of late autumn passerines, this bird exhibited almost Bonelli’s Warbler-like tones (Mark Pearson)

Observing freshly-arrived leaf warblers flitting among low-level clifftop vegetation in thick mist and drizzle in autumn is just about as evocative of classic east coast Observatory birding as it gets, and with the peak period for such events now upon us, it seems like a good time to keep such variation in mind – we’ll be paying close attention to similarly interesting individuals this year, and will update these pages with any comparable material.