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The UK Bird Atlas

Western Bonelli's Warbler (Phylloscopus bonelli)Where and when to see it in the UK

Passage migrant · reporting peaks September · Leaf Warblers · BOU category A

Records to 2026-08-16 · build 2026-08-31

Western Bonelli's Warbler (Phylloscopus bonelli)
Pierre Dalous, CC BY-SA 3.0, via Wikimedia Commons
Peak months
Western Bonelli's Warbler: relative reporting rate by 10 km square, September

Contains OS data © Crown copyright and database right 2026 (grid squares, place names from OS Open Names). Northern Ireland place names: OSNI Open Data, OGL v3.0. County and country boundaries: Office for National Statistics, Open Government Licence v3.0. Coastline: Natural Earth. Bird records from the data partners listed here.

Western Bonelli's Warbler has been recorded in 39 of the 2,971 10 km squares covering UK land (1%) in records to 2026. Passage migrant in the UK. Its strongest squares are in Scotland, England and Wales. Reporting peaks in September.

Conservation status: Least Concern (IUCN Red List, LC)
View on the IUCN Red List
Relative to this species' best squares (how classes are calculated)
Very high
100% or more of visits
High
2% to 100%
Moderate
<1% to 2%
Low
under <1%
Surveyed, not recorded
at least 20 qualifying visits, no record
Insufficient records
too few visits to say
Lighter fill
Few visits, lower confidence
Small dot
Occasional: a single record, or out of season

Seasonality

Reporting rate by month, relative to the peak month
MonthRelative to peakShare of visitsTypical class
January 0%<1%Insufficient records
February 0%<1%Insufficient records
March 0%<1%Insufficient records
April 1%<1%Low
May 0%<1%Very high
June 3%<1%Low
July 25%<1%Moderate
August 31%<1%Low
September 100%<1%Very high
October 79%<1%Moderate
November 0%<1%Insufficient records
December 0%<1%Insufficient records

Squares with the highest reporting rate in September

SquareWhereShare of visitsClass
HP61Shetland Islands, Scotland100%Very high
HU40Shetland Islands, Scotland100%Very high
HU41Shetland Islands, Scotland100%Very high
HU43Shetland Islands, Scotland100%Very high
HU56Shetland Islands, Scotland100%Very high
ND49Orkney Islands, Scotland100%Very high
NF70Na h-Eileanan Siar, Scotland100%Very high
SH12Gwynedd, Wales100%Very high
SS14Devon, England100%Very high
TG04Norfolk, England100%Very high

Spotted an error in a square? Report it and it goes into the review queue. Read the methodology for how classes, flags and sensitive-species rules work. Data to 2026.