There are two clearly differentiated races that lie at opposite ends of the species range, the main differences between them involve size and colour. The nominate form 'trochilus' occurring in the south-west are on average smaller and more olive-green and yellow, 'yakutensis' occurring in the east are generally larger and more pale brown and white. The group lying in between are named 'acredula', but these populations are highly variable, ranging from pale olive-green and typically 'acredula' to brown and white forms 'eversmannii'. These two forms and many intermediates, occur sometimes side by side in the north of range of 'acredula'.

 

There have been two Northern Willow Warblers reported from S. Gloucestershire;

 

1932: (1)

A presumed adult was ‘obtained’ sometime in April near Stoke Gifford.

 

1913: (1)

One, a presumed adult was 'obtained' sometime in May near Thornbury.