BirdTrends 2011 - BTO - British Trust for Ornithology

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Willow Warbler
Phylloscopus trochilus

Key facts

Conservation listings: Europe: no SPEC category (not concentrated in Europe, conservation status favourable) (BiE04)
UK: amber (species level, 25-50% population decline; race trochilus, 25-50% population decline, European status) (BoCC3)
Long-term trend: England: rapid decline
Population size: 2,125,000 territories in 2000 (1988-91 Atlas estimate updated using CBC/BBS trend: BiE04, APEP06)

http://www.bto.org/birdtrends2010/images/wilwa10TH300w.jpg

Status summary

Willow Warbler abundance has shown regionally different trends within the UK (Morrison et al. 2010). 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 was moved from the green to the amber list. This decline occurred mainly in southern Britain, 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 an initially upward trend in Scotland and in Northern Ireland, and continued 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 shallow decline in productivity as measured by CES and with a substantial increase in failure rates at the egg stage, which raises NRS concern (Leech & Barimore 2008). There is a small but significant decrease in the number of fledglings per breeding attempt. Average laying dates have become a week earlier, perhaps in response to recent climatic warming (Crick & Sparks 1999). Numbers have shown widespread moderate decrease across Europe since 1980 (PECBMS 2011a).

Population changes in detail

Source	Period (yrs)	Years	Plots (n)	Change (%)	Lower limit	Upper limit	Alert	Comment
CBC/BBS England	42	1967-2009	450	-60	-71	-49	>50
	25	1984-2009	642	-62	-68	-56	>50
	10	1999-2009	956	-27	-32	-23	>25
	5	2004-2009	1005	1	-3	6
CES adults	25	1984-2009	89	-68	-75	-60	>50
	10	1999-2009	91	-44	-52	-35	>25
	5	2004-2009	86	-9	-19	4
CES juveniles	25	1984-2009	92	-42	-63	-14	>25
	10	1999-2009	98	-10	-30	8
	5	2004-2009	93	14	-9	41
BBS UK	14	1995-2009	1366	-5	-11	3
	10	1999-2009	1413	-9	-13	-4
	5	2004-2009	1530	13	9	18
BBS England	14	1995-2009	900	-30	-37	-24	>25
	10	1999-2009	908	-23	-29	-18
	5	2004-2009	962	4	-1	9
BBS Scotland	14	1995-2009	209	21	6	35
	10	1999-2009	215	-4	-14	5
	5	2004-2009	249	11	1	22
BBS Wales	14	1995-2009	157	-10	-22	4
	10	1999-2009	172	-4	-12	7
	5	2004-2009	179	25	16	36
BBS N.Ireland	14	1995-2009	77	84	41	113
	10	1999-2009	88	17	2	37
	5	2004-2009	94	43	33	61

Tables show changes with their 90% confidence limits. Alerts are flagged for significant changes only. See here for more information.

Demographic trends

More on demographic trends

Variable	Period (yrs)	Years	Mean annual sample	Trend	Modelled in first year	Modelled in 2009	Change	Alert	Comment
Fledglings per breeding attempt	41	1968-2009	32	Linear decline	3.49 fledglings	3.01 fledglings	-14.0%
Clutch size	41	1968-2009	48	None
Brood size	41	1968-2009	135	None
Nest failure rate at egg stage	41	1968-2009	66	Linear increase	0.94% nests/day	1.70% nests/day	80.9%
Nest failure rate at chick stage	41	1968-2009	122	Linear increase	1.53% nests/day	2.02% nests/day	32.0%
Laying date	41	1968-2009	83	Linear decline	May 20	May 13	-7 days
Juvenile to Adult ratio (CES)	25	1984-2009	98	Smoothed trend	101 Index value	100 Index value	-1%
Juvenile to Adult ratio (CES)	10	1999-2009	105	Smoothed trend	71 Index value	100 Index value	41%
Juvenile to Adult ratio (CES)	5	2004-2009	99	Smoothed trend	77 Index value	100 Index value	29%

For details of analytical methods for the Nest Record Scheme, the Constant Effort Sites (CES) and the Retrapping Adults for Survival (RAS) scheme, please follow links here.