BTO - Breeding Birds of the Wider Countryside: Willow Warbler

WILLOW WARBLER
Phylloscopus trochilus

Willow Warbler © Tommy Holden

• Population
changes

• Productivity
trends

• Additional
information

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

UK population size

2,125,000 territories in 2000 (1988–91 Atlas estimate updated using CBC/BBS trend: BiE04, APEP06)

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, accompanied by 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. Pressures on migration and in the winter are likely to be affecting the population, as is a reduction in habitat quality on the breeding grounds (Fuller et al. 2005). The recent population decline is associated with a moderate decline in productivity as measured by CES and a substantial increase in failure rates at the egg stage. Average laying dates have become a week earlier, perhaps in response to recent climatic warming (Crick & Sparks 1999). Numbers have fallen widely in Europe since 1980 (PECBM 2006).

Population changes

Table of population changes for Willow Warbler

Source	Period (yrs)	Years	Plots (n)	Change (%)	Lower limit	Upper limit	Alert
CBC/BBS England	38	1967-2005	383	-60	-71	-48	>50
	25	1980-2005	499	-57	-65	-49	>50
	10	1995-2005	931	-31	-35	-26	>25
	5	2000-2005	816	-21	-26	-19
CES adults	21	1984-2005	88	-60	-68	-50	>50
	10	1995-2005	95	-45	-51	-39	>25
	5	2000-2005	82	-24	-30	-16
CES juveniles	21	1984-2005	90	-70	-77	-62	>50
	10	1995-2005	100	-51	-61	-42	>50
	5	2000-2005	90	-19	-31	-9
BBS UK	10	1995-2005	1251	-16	-22	-10
	5	2000-2005	1221	-15	-20	-11
BBS England	10	1995-2005	850	-34	-39	-30	>25
	5	2000-2005	797	-21	-26	-17
BBS Scotland	10	1995-2005	179	5	-10	18
	5	2000-2005	170	-11	-19	-2
BBS Wales	10	1995-2005	150	-28	-38	-16	>25
	5	2000-2005	166	-18	-26	-12
BBS N.Ireland	10	1995-2005	68	28	-3	47
	5	2000-2005	83	-14	-26	-3

Productivity trends

Table of productivity changes for Willow Warbler

Variable	Period (yrs)	Years	Mean annual sample	Trend	Modelled in first year	Modelled in 2005	Change
Clutch size	37	1968-2005	49	None
Brood size	37	1968-2005	132	None
Daily failure rate (eggs)	37	1968-2005	69	Curvilinear	1.08% nests/day	2.15% nests/day	99.1%
Daily failure rate (chicks)	37	1968-2005	121	None
Laying date	37	1968-2005	84	Linear decline	May 20	May 14	-6 days
Juvenile to Adult ratio (CES)	21	1984-2005	96	Smoothed trend	162 Index value	100 Index value	-38% >25
Juvenile to Adult ratio (CES)	10	1995-2005	106	Smoothed trend	122 Index value	100 Index value	-18%
Juvenile to Adult ratio (CES)	5	2000-2005	95	Smoothed trend	110 Index value	100 Index value	-9%

Additional information


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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., Barimore, C., Coombes, R.H., Downie, I.S., Freeman, S.N., Joys, A.C., Leech, D.I., Raven, M.J., Robinson, R.A. & Thewlis, R.M. (2007) Breeding Birds in the Wider Countryside: their conservation status 2007. BTO Research Report No. 487. BTO, Thetford. (http://www.bto.org/birdtrends) Pages maintained by Susan Waghorn and Iain Downie: Last updated 3 January, 2008