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Breeding Bird Survey
Common Birds Census
Constant Effort Sites
Nest Record Scheme
Waterways Bird Survey
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Summary
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4. DISCUSSION

4.5       Changes in breeding performance

Changes in a range of aspects of breeding performance can be measured by the Nest Record Scheme and the Constant Effort Sites scheme.  The former provides information on components of breeding performance per nesting attempt, while the latter provides an index of breeding performance accrued over all nesting attempts in a particular year, combined with the effect of changes in the survival of fledglings once they have left the nest and before they are caught as juveniles – a period when losses of young can be high.

Breeding performance can change for a wide variety of reasons, such as changes in food supply for parents and young, changes in predation pressure, changes in weather.  In some cases, declines or improvements in breeding performance may help to drive or be the prime factor in driving population changes.  Conversely, breeding performance may change in an apparently contradictory direction to the changes in population size: breeding performance may improve as populations decline or breeding performance may decline as populations increase in size.  These changes may be the result of so-called “density dependent” changes, in which increased crowding causes increased competition for resources and hence declines in breeding performance, and vice versa.  Alternatively, such changes may result from the loss of birds from poorer areas as populations decline, or the colonisation of poorer areas as populations increase, such that overall breeding performance changes.

4.5.1 Changes in clutch and brood size

The species showing statistically significant trends in clutch and brood size over the past 30 years (out of the 72 tested) are shown in Tables 4.5.1.1 and 4.5.1.2. Although the numbers of species showing increases and decreases in clutch size were approximately equal (14 vs. 11 species respectively) there were many more species showing increases in brood size than decreases over the same period (28 vs. 5).

Table 4.5.1.1 Significant trends in clutch size

Species

Period
(yrs)

Mean
annual
sample

Trend

Predicted
in first year

Predicted
in last year

Change

Comment

Long-tailed Tit

30

32

Linear decline

7.62 eggs

6.72 eggs

-0.9 eggs

 

Mute Swan

30

19

Linear decline

5.88 eggs

5.09 eggs

-0.79 eggs

Small sample

Rook

30

12

Linear decline

4.05 eggs

3.32 eggs

-0.73 eggs

Small sample

Hen Harrier

30

13

Linear decline

5.09 eggs

4.44 eggs

-0.65 eggs

Small sample

Magpie

30

52

Curvilinear

5.55 eggs

4.91 eggs

-0.64 eggs

 

Peregrine

30

16

Linear decline

3.69 eggs

3.11 eggs

-0.58 eggs

Small sample

Moorhen

30

98

Linear decline

6.52 eggs

6.08 eggs

-0.44 eggs

 

Common Sandpiper

30

13

Linear decline

3.96 eggs

3.75 eggs

-0.21 eggs

Small sample

Crow
30
1968-1998 Linear decline
4.02 eggs
3.84 eggs
-0.18 eggs
 

Pied Wagtail

30

61

Linear decline

5.12 eggs

4.98 eggs

-0.14 eggs

 

Nightjar

27

16

Linear decline

1.99 eggs

1.91 eggs

-0.08 eggs

Small sample

Whitethroat

30

26

Curvilinear

4.59 eggs

4.53 eggs

-0.06 eggs

Small sample

Sparrowhawk

30

44

Curvilinear

4.37 eggs

4.35 eggs

-0.02 eggs

 

Lapwing

30

131

Linear increase

3.69 eggs

3.81 eggs

0.12 eggs

 

Yellowhammer

30

44

Linear increase

3.38 eggs

3.53 eggs

0.15 eggs

 

Mistle Thrush

30

40

Linear increase

3.87 eggs

4.05 eggs

0.18 eggs

 

Grey Wagtail

30

43

Linear increase

4.8 eggs

5.01 eggs

0.21 eggs

 

Dunnock

30

104

Linear increase

3.91 eggs

4.18 eggs

0.27 eggs

 

Whinchat

30

12

Linear increase

5.4 eggs

5.72 eggs

0.32 eggs

Small sample

Skylark

30

43

Linear increase

3.34 eggs

3.69 eggs

0.35 eggs

 

Starling

30

86

Linear increase

4.44 eggs

4.8 eggs

0.36 eggs

 

Wren

30

102

Curvilinear

5.64 eggs

6 eggs

0.36 eggs

 

Stonechat

30

20

Linear increase

4.97 eggs

5.36 eggs

0.39 eggs

Small sample

Redstart

30

52

Linear increase

5.99 eggs

6.46 eggs

0.47 eggs

 

Tree Sparrow

30

93

Linear increase

4.77 eggs

5.36 eggs

0.59 eggs

 

Barn Owl

30

13

Linear increase

4.48 eggs

5.09 eggs

0.61 eggs

Small sample

See Help for information on what the categories mean.

For 10 species, significant changes in brood and clutch size were in the same direction (Mute Swan and Long-tailed Tit, negative; Yellowhammer, Dunnock, Stonechat, Skylark, Grey Wagtail, Redstart, Tree Sparrow, and Wren, all positive).  For three species, declines in clutch size were partially (Rook and Magpie) or fully (Moorhen) cancelled out by increases in average brood size, suggesting that conditions for young had improved for these species, although maybe conditions for the parents during egg-formation had declined.

Table 4.5.1.2 Significant trends in brood size

Species

Period
(yrs)

Mean
annual
sample

Trend

Predicted
in first year

Predicted
in last year

Change

Comment

Mute Swan

30

34

Curvilinear

4.47 chicks

3.74 chicks

-0.73 chicks

 

Great Tit

30

163

Linear decline

7.36 chicks

6.75 chicks

-0.61 chicks

 

Long-tailed Tit

30

27

Curvilinear

6.84 chicks

6.27 chicks

-0.57 chicks

Small sample

Yellow Wagtail

30

13

Linear decline

4.85 chicks

4.4 chicks

-0.45 chicks

Small sample

Great Spotted Woodpecker

30

14

Curvilinear

3.14 chicks

2.73 chicks

-0.41 chicks

Small sample

Wheatear

30

66

Curvilinear

4.72 chicks

4.75 chicks

0.03 chicks

 

Collared Dove

30

67

Linear increase

1.76 chicks

1.83 chicks

0.07 chicks

 

Swallow

30

303

Linear increase

4.13 chicks

4.25 chicks

0.12 chicks

 

Willow Warbler

30

142

Curvilinear

5.24 chicks

5.38 chicks

0.14 chicks

 

Red-throated Diver

18

42

Linear increase

1.25 chicks

1.44 chicks

0.19 chicks

 

Chaffinch

30

143

Linear increase

3.61 chicks

3.8 chicks

0.19 chicks

 

Spotted Flycatcher

30

137

Linear increase

3.64 chicks

3.86 chicks

0.22 chicks

 

Yellowhammer

30

69

Curvilinear

2.96 chicks

3.19 chicks

0.23 chicks

 

Linnet

30

127

Linear increase

4.12 chicks

4.36 chicks

0.24 chicks

 

Dunnock

30

110

Linear increase

3.42 chicks

3.67 chicks

0.25 chicks

 

Stonechat

30

52

Linear increase

4.65 chicks

4.94 chicks

0.29 chicks

 

Skylark

30

75

Linear increase

3.13 chicks

3.45 chicks

0.32 chicks

 

Jackdaw

30

75

Linear increase

2.75 chicks

3.08 chicks

0.33 chicks

 

Kestrel

30

113

Linear increase

3.82 chicks

4.16 chicks

0.34 chicks

 

Merlin

30

56

Linear increase

3.44 chicks

3.79 chicks

0.35 chicks

 

Tree Pipit

30

29

Linear increase

4.33 chicks

4.74 chicks

0.41 chicks

Small sample

Grey Wagtail

30

88

Linear increase

4.06 chicks

4.5 chicks

0.44 chicks

 

Dipper

30

151

Linear increase

3.49 chicks

3.95 chicks

0.46 chicks

 

Rook

30

98

Linear increase

2.28 chicks

2.82 chicks

0.54 chicks

 

Redstart

30

91

Curvilinear

5.09 chicks

5.63 chicks

0.54 chicks

 

Sparrowhawk

30

85

Linear increase

3.37 chicks

3.93 chicks

0.56 chicks

 

Magpie

30

88

Curvilinear

3.19 chicks

3.77 chicks

0.58 chicks

 

Corn Bunting

30

12

Curvilinear

3.09 chicks

3.7 chicks

0.61 chicks

Small sample

Tree Sparrow

30

104

Linear increase

3.83 chicks

4.49 chicks

0.66 chicks

 

Starling

30

222

Curvilinear

3.24 chicks

4.13 chicks

0.89 chicks

 

Moorhen

30

80

Curvilinear

3.51 chicks

4.43 chicks

0.92 chicks

 

Wren

30

130

Curvilinear

3.82 chicks

4.92 chicks

1.1 chicks

 

Nuthatch

30

56

Curvilinear

4.06 chicks

5.61 chicks

1.55 chicks

 

See Help for information on what the categories mean.

Changes that might be helping to drive the population changes are:

  • Decreased clutch and population size: Pied Wagtail, although the change is relatively small.
  • Decreased brood and population size: Yellow Wagtail – the decline in average brood size of nearly half a chick per nesting attempt is potentially important.  This is an aspect that the upcoming BTO project on Yellow Wagtails will need to investigate.
  • Increased clutch and population size: Redstart.
  • Increased brood and population size: Three corvids are prominent here; Jackdaw, Rook and Magpie have all enjoyed increased average brood sizes, as has Sparrowhawk.  For the latter, the return of the species into areas of the eastern UK where populations of songbird prey are greater may be a factor in this change.  The Chaffinch has shown an increase in average brood size that may be link to its shallow population increase.  Finally the Nuthatch, which has been spreading its distribution northwards and has increased its abundance considerably, has enjoyed the largest increase in average clutch size of all species, over 1.5 extra young per nesting attempt. It would seem quite likely that this has helped to drive the population increase.

Density dependent changes in average clutch or brood sizes are suggested for 16 and 17 species respectively, i.e. sizes have increased as populations decreased or vice versa.  For a few species, long-term population data are not available and changes in clutch or brood size may be density dependent responses and therefore might be potential warnings of population declines.  For Stonechat, Whinchat and Wheatear, atlas data support the suggestion of population declines.

4.5.2 Changes in nest failure rates

Statistically significant trends in the daily nest failure rates at the egg and chick stage over the past 30 years are shown in Tables 4.5.2.1 and 4.5.2.2 (75 and 69 species, respectively, were analysed in total). Although there was only a small preponderance of species showing declines in failure rates at the chick stage (16 vs. 9 species with increasing failure rates), there were three times as many species showing declines in failure rates at the egg stage than increases over the same period (35 vs. 11).   Thus the general picture is of improving nesting success.

Table 4.5.2.1 Significant trends in egg-stage daily failure rate of nests

Species

Period
(yrs)

Mean
annual
sample

Trend

Predicted
in first year

Predicted
in last year

Change

Comment

Tree Pipit

30

12

Linear decline

0.0453 nests/day

0.0104 nests/day

-0.0349 nests/day

Small sample

Woodlark

30

16

Linear decline

0.047 nests/day

0.0127 nests/day

-0.0343 nests/day

Small sample

Jay

30

11

Linear decline

0.0542 nests/day

0.0234 nests/day

-0.0308 nests/day

Small sample

Redshank

30

32

Linear decline

0.0425 nests/day

0.0182 nests/day

-0.0243 nests/day

 

Long-tailed Tit

30

51

Curvilinear

0.0316 nests/day

0.0085 nests/day

-0.0231 nests/day

 

Magpie

30

59

Linear decline

0.0278 nests/day

0.0047 nests/day

-0.0231 nests/day

 

Dipper

30

109

Linear decline

0.0225 nests/day

0.0023 nests/day

-0.0202 nests/day

 

Yellowhammer

30

66

Curvilinear

0.0517 nests/day

0.0317 nests/day

-0.02 nests/day

 

Wheatear

30

22

Linear decline

0.0219 nests/day

0.0077 nests/day

-0.0142 nests/day

Small sample

Treecreeper

30

25

Linear decline

0.0206 nests/day

0.0071 nests/day

-0.0135 nests/day

Small sample

Snipe

30

18

Linear decline

0.032 nests/day

0.0189 nests/day

-0.0131 nests/day

Small sample

Crow
30
56
Linear decline
0.0161 nests/day
0.0034 nests/day
-0.0127 nests/day
 

Robin

30

187

Curvilinear

0.0248 nests/day

0.0131 nests/day

-0.0117 nests/day

 

Song Thrush

17

375

Linear decline

0.0417 nests/day

0.033 nests/day

-0.0087 nests/day

 

Tawny Owl

30

54

Linear decline

0.0101 nests/day

0.0023 nests/day

-0.0078 nests/day

 

Starling

30

131

Linear decline

0.0114 nests/day

0.0041 nests/day

-0.0073 nests/day

 

Redstart

30

78

Linear decline

0.0115 nests/day

0.0044 nests/day

-0.0071 nests/day

 

Marsh Tit

30

19

Linear decline

0.0082 nests/day

0.0018 nests/day

-0.0064 nests/day

Small sample

Curlew

30

28

Curvilinear

0.03 nests/day

0.0237 nests/day

-0.0063 nests/day

Small sample

Jackdaw

30

48

Linear decline

0.0081 nests/day

0.0024 nests/day

-0.0057 nests/day

 

Kestrel

30

41

Linear decline

0.0064 nests/day

0.001 nests/day

-0.0054 nests/day

 

Pied Wagtail

30

84

Linear decline

0.0176 nests/day

0.0123 nests/day

-0.0053 nests/day

 

Barn Owl

30

11

Linear decline

0.0066 nests/day

0.0021 nests/day

-0.0045 nests/day

Small sample

Wren

30

151

Curvilinear

0.0171 nests/day

0.0129 nests/day

-0.0042 nests/day

 

Buzzard

30

23

Linear decline

0.0062 nests/day

0.0022 nests/day

-0.004 nests/day

Small sample

Tree Sparrow

30

122

Curvilinear

0.0069 nests/day

0.003 nests/day

-0.0039 nests/day

 

Stock Dove

30

62

Curvilinear

0.0116 nests/day

0.0079 nests/day

-0.0037 nests/day

 

Collared Dove

30

57

Curvilinear

0.0313 nests/day

0.0282 nests/day

-0.0031 nests/day

 

Sedge Warbler

30

49

Curvilinear

0.0147 nests/day

0.0117 nests/day

-0.003 nests/day

 

Sparrowhawk

30

40

Linear decline

0.0043 nests/day

0.0015 nests/day

-0.0028 nests/day

 

Great Tit

30

156

Linear decline

0.0062 nests/day

0.0037 nests/day

-0.0025 nests/day

 

Blue Tit

30

138

Linear decline

0.0048 nests/day

0.0029 nests/day

-0.0019 nests/day

 

Spotted Flycatcher

30

127

Curvilinear

0.0181 nests/day

0.0169 nests/day

-0.0012 nests/day

 

Dunnock

30

146

Curvilinear

0.0269 nests/day

0.026 nests/day

-0.0009 nests/day

 

Swallow

30

232

Curvilinear

0.0028 nests/day

0.0026 nests/day

-0.0002 nests/day

 

Raven

30

19

Curvilinear

0.0024 nests/day

0.0049 nests/day

0.0025 nests/day

Small sample

Moorhen

30

113

Curvilinear

0.0146 nests/day

0.0199 nests/day

0.0053 nests/day

 

Chaffinch

30

170

Curvilinear

0.0307 nests/day

0.0362 nests/day

0.0055 nests/day

 

Lapwing

30

143

Curvilinear

0.0175 nests/day

0.0232 nests/day

0.0057 nests/day

 

Oystercatcher

30

112

Linear increase

0.0129 nests/day

0.0205 nests/day

0.0076 nests/day

 

Linnet

30

160

Linear increase

0.0175 nests/day

0.0275 nests/day

0.01 nests/day

 

Ringed Plover

30

131

Curvilinear

0.0295 nests/day

0.0439 nests/day

0.0144 nests/day

 

Red-throated Diver

18

17

Linear increase

0.005 nests/day

0.0199 nests/day

0.0149 nests/day

Small sample

Reed Bunting

30

57

Linear increase

0.0067 nests/day

0.0257 nests/day

0.019 nests/day

 

Mute Swan

30

26

Curvilinear

0.0086 nests/day

0.0556 nests/day

0.047 nests/day

Small sample

Rook

30

39

Curvilinear

0.0175 nests/day

0.0683 nests/day

0.0508 nests/day

 
See Help for information on what the categories mean.

For 10species, significant changes in egg and chick failure rates were in the same direction (Linnet, positive; Woodlark, Magpie, Yellowhammer, Crow, Jackdaw, Starling, Pied Wagtail, Barn Owl and Wheatear, all negative).  For four species, declines in egg-stage failure rates were partially (Dipper and Long-tailed Tit) or fully (Wren and Spotted Flycatcher) cancelled out by increases in chick-stage failure rates, suggesting that opposing factors may affect nest success at the different stages for these species.

Table 4.5.2.2 Significant trends in chick-stage daily failure rate of nests

Species

Period
(yrs)

Mean
annual
sample

Trend

Predicted
in first year

Predicted
in last year

Change

Comment

Grey Heron

30

26

Linear decline

0.0688 nests/day

0.0008 nests/day

-0.068 nests/day

Small sample

Corn Bunting

30

12

Linear decline

0.0367 nests/day

0.0109 nests/day

-0.0258 nests/day

Small sample

Meadow Pipit

30

70

Linear decline

0.0297 nests/day

0.0106 nests/day

-0.0191 nests/day

 

Ring Ouzel

30

16

Linear decline

0.0229 nests/day

0.0064 nests/day

-0.0165 nests/day

Small sample

Woodlark

30

22

Curvilinear

0.0537 nests/day

0.0374 nests/day

-0.0163 nests/day

Small sample

Magpie

30

57

Linear decline

0.0177 nests/day

0.0021 nests/day

-0.0156 nests/day

 

Bullfinch

30

35

Linear decline

0.0309 nests/day

0.016 nests/day

-0.0149 nests/day

 

Yellowhammer

30

51

Curvilinear

0.0479 nests/day

0.0385 nests/day

-0.0094 nests/day

 

Jackdaw

30

45

Linear decline

0.0121 nests/day

0.0037 nests/day

-0.0084 nests/day

 

Grey Wagtail

30

63

Curvilinear

0.0146 nests/day

0.0081 nests/day

-0.0065 nests/day

 

Reed Warbler

30

91

Linear decline

0.0177 nests/day

0.0117 nests/day

-0.006 nests/day

 
Crow
30
45
Linear decline
0.0064 nests/day
0.0027 nests/day
-0.0037 nests/day
 

Starling

30

156

Linear decline

0.0059 nests/day

0.0028 nests/day

-0.0031 nests/day

 

Pied Wagtail

30

91

Curvilinear

0.015 nests/day

0.0125 nests/day

-0.0025 nests/day

 

Barn Owl

30

38

Linear decline

0.0024 nests/day

0.0003 nests/day

-0.0021 nests/day

 

Wheatear

30

45

Curvilinear

0.0138 nests/day

0.013 nests/day

-0.0008 nests/day

 

Dipper

30

86

Curvilinear

0.0052 nests/day

0.0068 nests/day

0.0016 nests/day

 

Swallow

30

205

Linear increase

0.0025 nests/day

0.0052 nests/day

0.0027 nests/day

 

Spotted Flycatcher

30

113

Linear increase

0.0093 nests/day

0.0145 nests/day

0.0052 nests/day

 

Wren

30

104

Curvilinear

0.0094 nests/day

0.0154 nests/day

0.006 nests/day

 

Blackcap

30

35

Curvilinear

0.0248 nests/day

0.0309 nests/day

0.0061 nests/day

 

Linnet

30

113

Linear increase

0.0146 nests/day

0.022 nests/day

0.0074 nests/day

 

Willow Warbler

30

131

Linear increase

0.0143 nests/day

0.022 nests/day

0.0077 nests/day

 

Long-tailed Tit

30

35

Linear increase

0.0074 nests/day

0.0159 nests/day

0.0085 nests/day

 

Nightjar

27

20

Linear increase

0.0019 nests/day

0.0167 nests/day

0.0148 nests/day

Small sample

See Help for information on what the categories mean.

Density dependent changes in egg- or chick-stage failure rates are suggested for 19 and 11 species respectively, i.e. failure rates have increased as populations have increased or vice versa

Changes that might be helping to drive the population changes are:

  • Increased egg-stage failure rates and decreased population size: Lapwing, Linnet, and Reed Bunting.  For the first two species, studies have suggested that this is an important factor in their population declines (Peach et al. 1994; Siriwardena et al. 2000b) and for the last species, it has been suggested that breeding performance may be holding back population recovery (Peach et al. 1999). Moorhen is also a species of potential concern because of increases in egg-stage failure rates that are concurrent with population declines measured by the CBC on farmland.
  • Decreased egg-stage failure rates and increased population size: Corvids appear to have benefited from improvements in nesting success at the egg stage: Jay, Magpie, Crow and Jackdaw, as have Sparrowhawk and Buzzard.  Changes in persecution and the decline of the impact of organochlorine pesticides are likely to have been important factors for these species.  Woodlark populations have increased in recent years and it may be that sympathetic habitat management has helped to improve nesting success for this species.  Long-tailed Tit populations have been expanding considerably in recent years and, being a relatively early nester that has taken advantage of recent climate warming (Crick et al. 1997, Crick & Sparks 1999) improvements in breeding performance may have helped this species’ population to expand.  Four other insectivores, Great & Blue Tits, Robin and Redstart have shown population increases with improved nest success.   The improvements in the nesting success of Collared Dove and Stock Dove could have major impacts on population size, given the relatively large number of nesting attempts made by each species each year.
  • Decreased chick-stage failure rates and increased population size: Three corvids again feature here, Jackdaw and Magpie, as does the Woodlark, all enjoying declines in chick-stage failure rates. Grey Heron populations have shown a steady increase over the years, and improvements in chick-stage nest survival may have played a part in recent years, perhaps helped by the declining impact of organochlorine pesticides and improvements in water quality of riverine and standing waterbodies. Reed Warbler is a species that has expanded its range in the UK over the years, and the small improvement nest success at the chick stage may have played a part.

Three species show increased chick-stage failure rates and decreased population size, but BTO studies suggest that these are unlikely to have driven the population declines: Spotted Flycatcher, Linnet, Willow Warbler.

For a few species, long-term population data are not available and changes in nest failure rates may provide a potential warning of population declines, either because they have the potential to drive population decline (Red-throated Diver and Ringed Plover) or because they are the result of density dependent changes (Wheatear, Tawny Owl and Ring Ouzel). 

4.5.3 Changes in productivity from CES

The CES has only been in operation since 1983, so the changes in productivity shown in table 4.5.3 covers about half the time period of the Nest Record Scheme results.  Statistical significance is not available for these trends at present, although a good indication can be obtained by inspecting the confidence intervals for the annual indices that are presented on the individual species graphs.  Overall, 21 species show declines in productivity and only 7 show improvements. 

Eight of the declines in productivity are greater than 25% over 14 years and a further 3 are greater than 50%.  For three of these species, Redpoll, Spotted Flycatcher and Willow Warbler, there have been substantial population declines.  While for the latter two species, declines in nesting success are unlikely to have been a major factor in driving their population declines, the changes in productivity may represent declines in post-fledging survival, which could be a factor of some importance.  Very little is known about Redpoll populations, as it is a species not well covered by BTO population monitoring schemes: the large decline (-43%) in CES productivity should be of conservation concern.  The declines in productivity may be a factor holding back the recovery of Linnet, Reed Bunting and Whitethroat, which declined before the CES was initiated. In addition the large decline in Nightingale productivity, is of concern given the complex changes in its distribution shown by the 1999 survey, in which declines have been shown over large parts of its range. The importance of the substantial declines in productivity of Greenfinch, Blue Tit, Sedge Warbler and Garden Warbler is unclear at the moment, but warrant close attention.

Only two species show increases greater than 10%: Reed Warbler and Bullfinch, both of which have shown declines over the last 14 years.   These increases in productivity are also shown by the Nest Record Scheme and may reflect a density dependent response to population decline.

Table 4.5.3 Changes in productivity indices (Percentage juveniles) for CES 1984-1998 (14 years)

Species

Mean
annual
sample

Change

Comment

Nightingale

12

-70%[>50]

Small sample

Greenfinch

45

-64%[>50*]

 

Linnet

24

-56%[>50]

 

Lesser Redpoll

22

-43%[>25]

 

Spotted Flycatcher

25

-39%[>25]

 

Blue Tit

96

-39%[>25*]

 

Sedge Warbler

66

-39%[>25]

 

Garden Warbler

78

-37%[>25]

 

Willow Warbler

96

-36%[>25*]

 

Reed Bunting

60

-26%[>25]

 

Whitethroat

71

-25%[>25]

 

Song Thrush

85

-24%

 

Great Tit

94

-16%

 

Yellowhammer

26

-11%

 

Treecreeper

63

-11%

 

Robin

95

-8%

 

Blackcap

91

-8%

 

Blackbird

95

-8%

 

Goldfinch

35

-7%

 

Chaffinch

80

-6%

 

Willow Tit

40

-5%

 

Dunnock

95

1%

 

Long-tailed Tit

78

1%

 

Lesser Whitethroat

57

2%

 

Wren

96

4%

 

Chiffchaff

81

6%

 

Reed Warbler

57

12%

 

Bullfinch

85

13%

 
See Help for information on what the categories mean.

4.5.4 Changes in average laying dates

Laying dates have been getting earlier over the past 25 years for many species (Crick et al.1997) and have shown curvilinear responses over the past 50 years as spring temperatures have cooled and then warmed (Crick & Sparks 1999).  Table 4.5.4 confirms that over the past 30 years, the majority of species with significant trends show trends towards earlier laying (data for 63 species were analysed in total).  Thus 22 species are laying between 22 days and 2 days earlier, on average, than they were 30 years ago.  There are no taxonomic or ecological associations between the species showing such changes, as they seem to occur across the board (Crick et al. 1997). Only two species show significant changes towards later laying, both of which suffer from small sample sizes and appear to be driven by a small number of outlying late years toward the end of the time series.  It is likely that the laying dates of the majority of those 39 species that do not show a significant trend in timing of laying are related to some aspect of weather, but that those aspects do not show any trend over time (Crick & Sparks 1999).

The significance of the changes in phenology for breeding performance and productivity is currently unknown and needs to be investigated. Earlier average laying may be beneficial for birds because earlier fledging is often related to improved survival to the following year.  However, several studies are beginning to show that birds are unable to advance their phenology sufficiently to match phenological changes in their food supply, such that later nesting birds are suffering from poorer productivity.  Early nesting parents have an increased chance of having their offspring recruited into the next generation (Visser et al. 1998).  The conservation significance of factors such as these needs to be assessed urgently.

Table 4.5.4 Significant trends in laying date (Day 1 = 1 Jan) over 30 years (1968-1998)

NESPNUM

Mean
annual
sample

Trend

Predicted
in first year

Predicted
in last year

Change

Comment

Magpie

40

Curvilinear

day 110

day 88

-22 days

 

Corn Bunting

15

Earlier laying

day 181

day 165

-16 days

Small sample

Tree Pipit

17

Curvilinear

day 145

day 132

-13 days

Small sample

Chiffchaff

40

Curvilinear

day 135

day 123

-12 days

 

Long-tailed Tit

43

Curvilinear

day 108

day 96

-12 days

 

Greenfinch

99

Earlier laying

day 145

day 134

-11 days

 

Nuthatch

25

Earlier laying

day 122

day 113

-9 days

Small sample

Chaffinch

115

Curvilinear

day 129

day 121

-8 days

 

Oystercatcher

47

Earlier laying

day 137

day 130

-7 days

 

Dipper

65

Earlier laying

day 108

day 101

-7 days

 

Wren

92

Curvilinear

day 133

day 126

-7 days

 

Redstart

66

Curvilinear

day 140

day 133

-7 days

 

Ring Ouzel

27

Earlier laying

day 135

day 128

-7 days

Small sample

Marsh Tit

14

Earlier laying

day 118

day 111

-7 days

Small sample

Treecreeper

14

Earlier laying

day 127

day 120

-7 days

Small sample

Swallow

93

Curvilinear

day 170

day 164

-6 days

 

Meadow Pipit

44

Earlier laying

day 138

day 132

-6 days

 

Blackcap

36

Curvilinear

day 139

day 133

-6 days

 
Crow
36
Linear decline
day 108
day 102
-6 days
 

Willow Warbler

92

Earlier laying

day 139

day 136

-3 days

 

Jackdaw

20

Curvilinear

day 113

day 110

-3 days

Small sample

Reed Warbler

147

Curvilinear

day 166

day 164

-2 days

 

Skylark

23

Curvilinear

day 146

day 148

2 days

Small sample

Grey Heron

15

Curvilinear

day 99

day 110

11 days

Small sample

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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