BTO - Breeding Birds of the Wider Countryside: Willow Warbler

WILLOW WARBLER
Phylloscopus trochilus

Willow Warbler © Tommy Holden

Conservation listings

Europe: no SPEC category (favourable conservation status in Europe, not concentrated in Europe)
UK: amber (25-50% population decline)

Long-term trend

England: rapid decline

Distribution maps

Maps and statistics from British and Irish
atlases can be viewed here.

Status summary

Willow Warbler abundance has apparently shown different trends at different UK latitudes. The overall CBC/BBS trend shows a rapid decline during the 1980s and early 1990s, after 20 years of relative stability, and, on the strength of a 31% decline on CBC plots between 1974 and 1999, the species has been moved from the green to the amber list. This decline occurred mainly in the south of the UK, however, due to a fall in survival rates there (Peach et al. 1995a), with Scottish populations remaining unaffected. BBS figures since 1994 indicate a stark contrast between strong increase in Scotland and in Northern Ireland, and further severe decreases in England and in Wales. The recent population decline is associated with a moderate decline in productivity as measured by CES and increases in failure rates at the egg stage, although average brood size has increased. Laying dates have become earlier, perhaps in response to recent climatic warming (Crick & Sparks 1999).

Table of population changes for Willow Warbler

Source	Period (yrs)	Years	Plots (n)	Change (%)	Lower limit	Upper limit	Alert	Comment
CBC/BBS England	35	1967-2002	346	-58	-70	-45	>50
	25	1977-2002	425	-53	-63	-41	>50
	10	1992-2002	800	-29	-35	-22	>25
	5	1997-2002	987	-29	-31	-25	>25
CES adults	18	1984-2002	89	-60	.	.	*[>50]**
	10	1992-2002	101	-48	.	.	*[>25]**
	5	1997-2002	96	-43	.	.	*[>25]**
CES juveniles	18	1984-2002	89	-68	.	.	*[>50]**
	10	1992-2002	103	-49	.	.	*[>25]**
	5	1997-2002	102	-38	.	.	*[>25]**
BBS UK	9	1994-2003	1191	-11	-15	-7
BBS England	9	1994-2003	823	-37	-41	-33	(>25)
BBS Scotland	9	1994-2003	172	25	9	42
BBS Wales	9	1994-2003	136	-34	-42	-25	(>25)
BBS N.Ireland	9	1994-2003	57	47	9	99

Table of productivity information for Willow Warbler

Variable	Period (yrs)	Years	Mean annual sample	Trend	Predicted in first year	Predicted in last year	Change	Comment
Clutch size	34	1968-2002	50	None
Brood size	34	1968-2002	134	Curvilinear	5.24 chicks	5.5 chicks	0.26 chicks
Daily failure rate (eggs)	34	1968-2002	69	Curvilinear	0.0112 nests/day	0.0227 nests/day	0.0115 nests/day
Daily failure rate (chicks)	34	1968-2002	123	None
Laying date	34	1968-2002	86	Linear decline	day 140	day 135	-5 days
Percentage juveniles (CES)	18	1984-2002	96	Smoothed trend	142 productivity index	102 productivity index	*-29%[>25]**
Percentage juveniles (CES)	10	1992-2002	110	Smoothed trend	105 productivity index	102 productivity index	-4%
Percentage juveniles (CES)	5	1997-2002	109	Smoothed trend	93 productivity index	102 productivity index	8%


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This report should be cited as: Baillie, S.R., Marchant, J.H., Crick, H.Q.P., Noble, D.G., Balmer, D.E., Beaven, L.P., Coombes, R.H., Downie, I.S., Freeman, S.N., Joys, A.C., Leech, D.I., Raven, M.J., Robinson, R.A. and Thewlis, R.M. (2005) Breeding Birds in the Wider Countryside: their conservation status 2004. BTO Research Report No. 385. BTO, Thetford. (http://www.bto.org/birdtrends) Pages maintained by Susan Waghorn and Iain Downie: Last updated 18 January, 2006