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BTO Blueline
WILLOW WARBLER
Phylloscopus trochilus

Willow Warbler © Tommy Holden

Conservation listings

Europe: no SPEC category, secure
UK: Amber (25-50% population decline)

Long-term trend
England: moderate decline
 
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 now 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. 1995), with Scottish populations remaining unaffected. BBS figures since 1994 indicate a stark contrast between strong increase in Scotland and Northern Ireland, and further 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 both egg and clutch stages. Laying dates have become earlier, perhaps in response to recent climatic warming (Crick & Sparks 1999).

 
CBC/BBS England graph
 
Table of population changes for Willow Warbler
Source Period
(yrs)
Years Plots
(n)
Change
(%)
Lower
limit
Upper
limit
Alert Comment
CBC/BBS England 33 1967-2000 336 -49 -63 -32 >25  
  25 1975-2000 396 -44 -55 -32 >25  
  10 1990-2000 693 -30 -39 -24 >25  
  5 1995-2000 1030 -12 -15 -8    
CES adults 17 1984-2001 90 -54 . . [>50*]  
  10 1991-2001 104 -42 . . [>25*]  
  5 1996-2001 103 -35 . . [>25*]  
CES juveniles 17 1984-2001 89 -64 . . [>50*]  
  10 1991-2001 104 -47 . . [>25*]  
  5 1996-2001 106 -36 . . [>25*]  
BBS UK 8 1994-2002 1202 -8 -13 -4    
BBS England 8 1994-2002 840 -33 -36 -29 (>25)  
BBS Scotland 8 1994-2002 172 25 9 43    
BBS Wales 8 1994-2002 134 -28 -36 -18 (>25)  
BBS N.Ireland 8 1994-2002 51 47 7 101    
BBS acknowledgement
 
CES adult abundance
CES adult abundance
BBS UK graph
BBS England graph
BBS Scotland graph
BBS Wales graph
BBS N Ireland graph
 
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 33 1968-2001 52 None        
Brood size 33 1968-2001 139 Curvilinear 5.24 chicks 5.49 chicks 0.25 chicks  
Daily failure rate (eggs) 33 1968-2001 72 Curvilinear 0.0114 nests/day 0.0219 nests/day 0.0105 nests/day  
Daily failure rate (chicks) 33 1968-2001 128 Linear increase 0.015 nests/day 0.0207 nests/day 0.0057 nests/day  
Laying date 33 1968-2001 89 Linear decline day 140 day 135 -5 days  
Percentage juveniles (CES) 17 1984-2001 96 Smoothed trend 147 productivity index 101 productivity index -32%[>25*]  
Percentage juveniles (CES) 10 1991-2001 112 Smoothed trend 112 productivity index 101 productivity index -10%  
Percentage juveniles (CES) 5 1996-2001 113 Smoothed trend 102 productivity index 101 productivity index -2%  
 
Clutch size graph
Brood size graph
Egg nest failure graph
Chick nest failure graph
Laying date graph
CES productivity graph
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This report should be cited as:
Crick, H.Q.P., Marchant, J.H., Noble, D.G., Baillie, S.R., 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. (2004)
Breeding Birds in the Wider Countryside: their conservation status 2003.
BTO Research Report No. 353. BTO, Thetford. (http://www.bto.org/birdtrends2003)

Pages maintained by Susan Waghorn and Iain Downie: Last updated 16 February, 2009