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| 4.3 |
Alerts
over 25, 10 and 5 years |
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| The Common
Bird Census (CBC) population trends discussed in Section
4.2 were calculated using all CBC data collected over the 31-year period
between 1968 and 1999. Population trends may also be calculated, and alerts may
be raised, over shorter time periods. Alerts are calculated 25 years retrospectively
in order to allow direct comparison with those used in the original conservation
listing process. The additional calculation of trends during the periods 10 and
5 years prior to the current year allows rapid declines in population sizes to
be identified quickly, before numbers of individuals fall to such a level that
reversal of the trend becomes very difficult. |
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| 4.3.1 |
Common
Birds Census Alerts |
| There are relatively
few major differences between the alerts raised over 25 years and those raised
over 31 years which have already been discussed. Five additional species raise
alerts over 25 years. No alert is raised for a sixth species, Red-legged Partridge,
although numbers have declined by >25%, as the size of the population is severely
influenced by the release of captive individuals for shooting. |
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- Kestrel (31 years,
-4%, non-signficant; 25 years, -28%, significant): Discussed in section
4.2.3.
- Curlew (31 years,
-30%, non-significant; 25 years, -38%, significant): Curlew populations in lowland
Britain have declined steadily since the mid-1970s, probably due to a range contraction
resulting from the drainage of farmland habitats. Although the CBC does not provide
good coverage of the UK's breeding population, the Breeding Birds Survey has also
identified recent population declines.
- Lesser Spotted Woodpecker
(31 years, -60%, non-significant; 25 years, -73%, significant): This species experienced
a population increase in the late 1960s and early 1970s, followed by sustained
decline. The increase may have been due to increases in the amount of dead wood
available owing to the effects of Dutch Elm Disease, although the decline is similar
to that exhibited by a variety of other woodland specialists, such as Marsh and
Willow Tits.
- Meadow Pipit (31
years, -30%, non-significant; 25 years, -43%, significant): The decline in Meadow
Pipit abundance indicated by the CBC may not be representative of the whole population,
as key habitats such as moorland are poorly monitored under this scheme. However,
the species has exhibited a significant range contraction in lowland England since
the mid 1970s.
- Goldcrest (31 years,
+15%, non-significant; 25 years, -55%, significant): Although this might be viewed
with some scepticism because its population is subject to large annual fluctuations
due to the weather, the smoothed population trend for this species demonstrates
a sustained decrease, which contrasts strongly with the population recoveries
displayed by two other small-bodied resident insectivores: Wren and Long-tailed
Tit. However, it should be noted that the CBC monitors relatively few pure conifer
woods and that most Goldcrests are recorded in relatively small numbers on plots
that consist mainly of non-conifer habitats.
- House Sparrow (31
years, -34%, non-significant; 25 years, -46%, significant): This species has been
incompletely monitored by CBC because of a strong urban component to its population
and because, prior to 1973, data were not gathered systematically. However, the
BTO's Garden Bird Feeding Survey also indicates large population declines in the
suburban population (Glue 1994).
|
| Complete tables
of those species triggering alerts at 25, 10 and 5 years are given in the Appendix
7.1. |
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| 4.3.2 |
Waterways
Bird Survey Alerts |
| The WBS provides
information concerning population changes over a maximum period of 24 years. Six
species trigger alerts over this time period (Table 4.3.2). |
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- Yellow Wagtail:
The decline of this species by 81% over 23 years is extremely serious and probably
reflects a deterioration in the suitability of any adjacent farmland for foraging,
perhaps combined with a deterioration in riverine habitat quality and management.
This supports evidence of a more widespread decline in this species, which has
been linked to the loss of wet meadows. The BTO, in conjunction with Anglia Water,
began an investigation into the ecology of this species in 2002.
- Reed Bunting: The
68% decline of this species along linear waterways reflects that measured by the
CBC in other habitats over a similar time period. Although the declining survival
rates are mainly responsible for the decrease in abundance, it is possible that
declines in breeding success might be preventing recovery (Peach
et al. 1999).
- Little Grebe: Although
the WBS does not monitor Little Grebes on still water bodies and the sample sizes
monitored are relatively small, the decline on linear waterways is considerable
(-56%) and suggests that an investigation of the potential cause of the decline
and of its ecology is required.
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| In addition
to these rapid declines, three species show declines of 25-49%: |
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- Pied Wagtail: Although
not generally considered to be a bird associated closely with linear water bodies,
this species is relatively common on WBS plots where the population has declined
by 48% over the past 24 years. This decline has not affected the majority of the
population, which occurs in drier habitats, but it may reflect a potentially important
decline in the condition of riparian habitats.
- Grey Wagtail: This
is the third wagtail species to exhibit substantial declines along linear waterways
(-41%). Grey Wagtail is the species most closely associated with rivers and streams,
feeding alongside and over them, and is perhaps the strongest indicator that a
serious decline in the quality of riparian habitat has occurred over the past
24 years.
- Redshank (>25%):
Although WBS data suggest that numbers of Redshank have declined along inland
waterways since the mid-1970s, data collected under the BBS, which provides a
more complete coverage of the species' range, show that there has been no clear
trend in abundance between 1994 and 2000. The size of the wintering population
also appears to be stable (Musgrove
et al. 2001), although many of these wintering birds do not belong
to the British population.
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| The decline
in waders on wet meadows is of some conservation concern. A resurvey
of sites surveyed in England & Wales was due to be carried out in 2001,
but has been postponed until 2002 because of the outbreak of foot-and-mouth disease
and consequent restrictions on access to the countryside. |
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| Table 4.3.2
Alerts for WBS waterways 1975-1999 |
|
|
See Help
for information on category definitions.
|
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| 4.3.3 |
Constant
Effort Sites Alerts |
| The majority
of species that trigger alerts from the CES over the last 15 years are also the
subject of alerts from the CBC. However, these alerts are useful because they
cover a very different set of habitats, including wet and dry scrub and reedbeds,
which are not represented by CBC. Thus >50% alerts are raised for Linnet,
Redpoll, Spotted Flycatcher and Yellowhammer, and >25% alerts
are raised for Reed Bunting, Song Thrush, Willow Tit, and
Willow Warbler, although it should be noted that the CES does not necessarily
monitor a representative sample of the populations of Spotted Flycatcher and Redpoll. |
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| Interestingly,
the CES also indicates a substantial decline (-33%) in Whitethroat abundance
that is not shown by CBC over a similar time period, perhaps confirming that this
species has not recovered from the sub-Saharan drought-induced decline of 1968. |
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| CES data trigger
an alert for one additional species: Lesser Whitethroat. This species has
declined by 43% over the past 15 years. Lesser Whitethroat are rather enigmatic
birds which winter in eastern Africa, in contrast to most of the UK's other long-distance
migrants that winter in western or southern Africa. Population declines may be
due to the influence of factors acting on the population on their wintering grounds. |
| |
| Comparison
between CES and CBC over the past 10 years indicates that some species have declined
much faster on CES than on CBC plots: This is especially true for Linnet,
which declined by 80% on CES plots but increased by 9% on CBC plots. Indeed, much
of this decline on CES plots has occurred over the past 5 years, with a 56% decline
over that period. Reed Bunting populations declined more rapidly on CES
plots (-39%) than on CBC plots (-25%) or WBS plots (-14%), which is worrying as
the CES reedbed and wet scrub habitats are likely to be the preferred habitat
for this species. Song Thrush populations have declined faster on CES plots
(-31%) than on CBC plots (-4%), as have Lesser Whitethroat populations
(CES -58%; CBC -17%). A more rapid decrease on CBC plots has only been indicated
for Redpoll (CES -67%; CBC -83%), Yellowhammer (CES -32%; CBC -39%),
and Willow Tit (CES -51%, CBC -64%). |
|
|
Table
4.3.3 Alerts for CES adults 1984-1999
|
|
|
See Help
for information on category definitions.
|
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| 4.3.4 |
Breeding
Bird Survey Population Changes |
| The BBS has
been designed to provide a properly representative coverage of the whole of the
UK. However, it has only been in operation since 1994, so population changes reported
here are all calculated over a six-year period. These measures of change have
been derived from simple annual indices and have not been subject to the same
analytical approaches (smoothing etc) as the longer-running schemes. The results
should therefore be interpreted with this limitation in mind. |
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| Several of
the species with population changes of greater than 25% on BBS sites in the UK
(as well as in individual countries) have been in long-term decline, as indicated
by CBC and WBS data: |
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- BBS - UK & England: Corn
Bunting and Willow Tit.
- BBS - UK & Scotland: Kestrel.
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| Several other
species that have exhibited long-term declines on CBC plots have decreased in
number by greater than 25% in a particular country, e.g. England, not in the UK
as a whole: |
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- BBS - England: Lesser
Redpoll, Cuckoo and Grey Partridge.
- BBS - Scotland: Lapwing.
- BBS - Wales: Starling
and Yellowhammer.
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| Species declines
that have not been identified by the more established schemes include: |
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- UK for: Wood Warbler
(and in England) and Shelduck.
- England for: Great Black-backed
Gull.
- Scotland for: Black-headed
Gull, Carrion Crow and Swift.
- Wales for: Mallard.
|
| For many
of these species, long-established BTO monitoring schemes may not have provided
sufficient coverage of their distributional ranges. The rapid declines reported
from BBS may therefore be important indicators of potentially new conservation
problems, although some declines may simply reflect temporary natural fluctuations
in population size caused by factors such as weather conditions. |
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| Details
of BBS population changes are given in Appendix 7.4. |
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|
Table
4.3.4 Population Changes for BBS UK 1994-2000
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See
Help for information on what the categories mean.
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