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A mystery solved ?– vagrancy in Siberian Phylloscopus warblers

        

Over wintering Yellow-Browed Warbler, Frampton on Severn, January 2025

One of the avian phenomena that perplexes me most is the yearly UK autumn arrival of significant numbers of Siberian Phylloscopus warblers. Every year I travel to see Yellow-Browed Warblers that are overwintering in the UK and wonder what on earth they are doing here.  This is a really perplexing one as these delightful little leaf warblers breed in the eastern palearctic and migrate to tropical and south-east Asia for the winter. So overwintering birds in the UK have flown some 5,000 km in the wrong direction in perhaps the largest UK example of reverse migration. One  intriguing possibility is that mother nature is using reverse migration to establish independent breeding colonies hence mitigating the risk of the species being wiped out by some environmental  catastrophe in their home breeding range.

 

I’ve ponder this perplexing question many times in my blogs, see here. I’ve also previously reviewed the excellent “Vagrancy in Birds” by Alexander Lees and James Gilroy which discusses the possible cause of vagrancy in a wide range of avian species.

 

A recent excellent scientific paper, “Using age-ratios to investigate the status of two Siberian Phylloscopus species in Europe” seems to have solved this conundrum. The authors studied records of overwintering Yellow-browed Warblers  and Siberian Chiffchaffs caught and ringed in western Europe. 

 

Quoting the paper

 

“Because migratory songbirds tend to follow the same orientation and migration route throughout their lives, age assessment can help to distinguish individuals which have already used a route (i.e. adults) from those using it for the first time (i.e. juveniles). The presence of a significant proportion of adult individuals would suggest the presence of regular migrants in a vagrancy context.”

 

They looked at 324 records of Yellow-browed Warblers caught at various locations over the past decade and found that only one was a definitive adult indicating that a new migration route is not being established. As migration route fidelity is very strong, this sadly suggests that none of these bird survive to migrate as adults.

 

The findings with Siberian Chiffchaffs were markedly different. Of the 117 records investigated 30% of overwintering birds were adults. Again assuming migration route fidelity, this  indicates that these birds have established a sustainable alternative winter migration route. Giving the difficulty of establishing pure breed Sibs from interbreeds DNA analysis was used to  establish their genetic purity. A pure Sib was defined as one having a hybrid score of 5% or less. They found 36 pure sibs with a score >95% and 19 individuals with a score <95%  suggesting these later birds have a history of some past hybridisation with Common Chiffchaffs. This raises the strong  possibility that new migration routes are being created by the presence of some Common Chiffchaff genes in the Sib population.

 

I hope I have summarise this excellent piece of scientific research correctly but If you find this interesting, I strongly suggest you read the original paper which has much more detail on the authors study and conclusions.


Footnote – my blogs are posted with sometimes rather imaginative spelling and grammar due to my extreme dyslexia!  

 

 

 

 

 

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