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4. DISCUSSION

4.4    Increasing species

Those species that have increased by more than 25% over the past 30 years on CBC plots and 23 years on WBS plots are shown in Tables 4.4.1 & 4.4.2 respectively. Three identifiable groups stand out: the corvids - Jackdaw, Crow and Magpie; the doves - Woodpigeon, Stock Dove and Collared Dove; and birds of prey - Sparrowhawk and Buzzard.Corvids appear to have benefited from relaxation and decrease of game keeping activities in the countryside in recent years and the doves have probably benefited from the increased use of brassica (particularly oilseed rape) crops. The birds of prey have been expanding with the decline of organochlorine pesticides in the environment (which affected productivity and survival) and have benefited from declines in persecution (e.g. Ratcliffe 1993).

There is a group of resident insectivores that has increased in population size. The majority are associated with woodland: Green Woodpecker and Great Spotted Woodpecker, Nuthatch, Blue Tit, Long-tailed Tit and, on farmland CBC plots, Great Tit and Coal Tit and Wren.The reasons for these changes are unclear.Pied Wagtail has increased on CBC plots over 30 years, but declined by 49% on WBS plots over the past 23 years – although neither surveys may be entirely representative of the UK’s population of this species.

In addition to the resident insectivores, there is a small group of migrant insectivores that have increased by greater than 50% in abundance: Blackcap and Reed Warbler on CBC over the past 30 years, and Reed Warbler on WBS over the past 23 years. Again the reasons for these changes are unclear.

Table 4.4.1 >25% population increases for CBC all habitats 1968-1998

Species

Period
(yrs)

Plots
(n)

Change
(%)

Lower
limit

Upper
limit

Alert

Comment

Blue Tit

30

216

35

21

48

Pheasant

30

126

36

9

87

Long-tailed Tit

30

129

53

13

111

Pied Wagtail

30

84

70

24

139

Unrepresentative

Jackdaw

30

<76

72

15

153

Crow

30

167

82

53

129

Woodpigeon

30

97

86

13

184

Coot

30

31

94

40

300

Unrepresentative?

Mallard

30

112

<101

68

150

Reed Warbler

30

24

103

43

253

Unrepresentative?

Blackcap

30

155

106

71

153

Magpie

30

157

107

74

154

Nuthatch

30

64

116

53

191

Green Woodpecker

30

79

118

74

202

Great Spotted Woodpecker

30

97

118

74

214

Little Grebe

30

15

149

10

899

Unrepresentative?
small sample

Stock Dove

30

75

157

63

293

Mute Swan

30

20

174

34

403

Unrepresentative?
small sample

Sparrowhawk

30

37

212

64

487

Buzzard

30

22

332

204

954

Unrepresentative?

Shelduck

30

18

349

128

743

Unrepresentative?
small sample

Collared Dove

30

71

1284

629

<2934

Tufted Duck

30

16

<2327

918

4838

Unrepresentative?
small sample

See Help for information on what the categories mean.

Finally there is a large group of birds associated with freshwater habitats.  For most of these the CBC and WBS cannot be said to provide monitoring of a representative portion of the population, but these results are interesting indicators of changes that may be affecting the whole populations.  We can be confident that Grey Heron populations have increased in England and Wales over the past 69 years and Mallard populations have increased on CBC and WBS plots. The increases recorded for Mute Swan on CBC and WBS are likely to be the result of the eradication of lead weights used by anglers. The reasons for increases of Coot, Tufted Duck and Shelduck are unclear, and the increase of Little Grebe on CBC plots contradicts the rapid decline shown on WBS plots, although neither scheme is likely to provide representative monitoring for a species that prefers still water bodies. Two waders have increased on WBS plots over the past 3 years: Curlew and Oystercatcher. The forthcoming Survey of Breeding Waders of Lowland Wet Meadows should provide more information on the lowland breeding populations of these species.

Table 4.4.2 >25% population increases for WBS waterways 1975-1998

Species

Period
(yrs)

Plots
(n)

Change
(%)

Lower
limit

Upper
limit

Alert

Comment

Coot

23

39

63

4

200

Mute Swan

23

43

67

21

156

Reed Warbler

23

19

71

19

246

Small sample

Curlew

23

20

77

16

436

Small sample

Oystercatcher

23

23

109

74

164

Mallard

23

93

190

116

286

See Help for information on what the categories mean.

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The report should be cited as: Baillie, S.R., Crick, H.Q.P., Balmer, D.E., Bashford, R.I., Beaven, L.P., Freeman, S.N., Marchant, J.H., Noble, D.G., Raven, M.J., Siriwardena, G.M., Thewlis, R. and Wernham, C.V. (2001) Breeding Birds in the Wider Countryside: their conservation status 2000. BTO Research Report No. 252. BTO, Thetford. (http://www.bto.org/birdtrends)

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