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| 4.2 |
The
31-year alerts |
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| The
population sizes of 24 species have declined by greater than
25% over the 31-year period from 1968-99, 17 of which have declined
by greater than 50% (see table 4.2.1 and 4.2.2).
The majority are these species are on the JNCC’s Conservation
Importance List and on the conservation Non-Governmental Organisations’
(NGO) Birds of Conservation Concern List (see section 6 for
a description of the categories). |
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| The species
which have not changed status are: Grey Partridge, Turtle Dove, Skylark, Song Thrush, Spotted Flycatcher, Tree Sparrow, Linnet, Bullfinch and Corn Bunting, all with declines of greater
than 50%; and Dunnock which declined by between 25%
and 49%. |
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| 4.2.1 |
Recent 50% alerts |
| Here
we highlight those species which are not on the current JNCC Conservation Importance
List or NGO Birds of Conservation Concern List as having a >50% decline (see
section 2.7). |
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- Whitethroat:
this species underwent a severe population crash between 1968-69 due to the failure
of rains in its sub-Saharan wintering grounds. It was missed by the original conservation
listing process because the time-frame did not include this period, but the population
has shown little sign of real recovery. This indicates, perhaps, that conditions have not improved on its
wintering grounds or that conditions have worsened on its breeding grounds. Such recovery as has occurred appears to be
greater on farmland (-37%), which is presumably the preferred habitat, than it
is on woodland CBC plots (-83%).
- Starling,
Willow and Marsh Tits: these species were previously included on the conservation
listings as having declined by 25-49%, but their declines have substantially worsened
since then. The Starling has declined
more on woodland CBC plots (-90%) than on farmland plots (-64%). The pattern and causes of the Starling
decline are currently under investigation as part of a DEFRA-funded study.
- Bullfinch:
The UK Bullfinch population has been declining rapidly in size since the mid-1970s,
albeit at a progressively slower rate. Although the species is on the 50% conservation
listing, the magnitude of the population decline as indicated by CBC data has
only recently exceeded 50%. The scale of the decline has been more rapid in farmland
areas (-65%) than it has on woodland plots (-39%). The exact cause of this decline
is not clear, but recent
research suggests that increased nest failure rates at the egg
stage caused by a reduction in habitat structural complexity in agricultural areas
may be partially responsible (Siriwardena
et al. 2001).
- Yellowhammer:
this is the latest farmland seed-eating species to decline, the size of the population
having fallen substantially in the 1990s after maintaining a stable population
throughout the 1970s and 1980s, a period during which many other seed-eating species
declined. The decline has been greater on woodland CBC
plots (-76%) than on farmland (-39%), which is presumably its preferred habitat.
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| Three other species
trigger 50% alerts, but it should be noted that the CBC does not necessarily provide
monitoring coverage of a representative sample of their populations. |
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- Woodcock: The Woodcock was previously put on the conservation
listings because of a 50% range contraction between the two breeding bird atlases
(Gibbons et
al. 1993). Although the CBC
does not cover the distributional range of this species well, its sizeable decline
in lowland England may necessitate further investigation. The BTO and the Game
Conservancy Trust will be carrying out a survey of this species in 2003.
- Tree
Pipit: this species was not included on previous conservation listings for
the same reason as the Redpoll but, again, this upland woodland bird has shown
substantial population declines in lowland England, which may justify an investigation
into its ecology.
- Redpoll:
this species was not included on previous conservation listings because the CBC
does not cover the centres of its distributional range, particularly in Scotland
and Wales. However, it has shown the second
largest population decline of all UK bird species studied, after the Tree Sparrow,
and indicates a potential problem in at least a part of its range (lowland England).
Furthermore, the population exhibited a substantial range contraction between
the two breeding bird atlases (Gibbons
et al. 1993). The causes of these declines are unknown and
certainly warrant further investigation.
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| Table
4.2.1 >50% population alerts for CBC all habitats 1968-1999 |
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|
See Help for information on category definitions.
|
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| Lesser
Spotted Woodpecker is not included in this table as, although the CBC indicates
that numbers have decreased by >50% over the last 31 years, this decline is
not statistically significant. |
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| 4.2.2 |
Recent
25% Alerts |
| Here we highlight
those species that are not listed as having a 25-49% decline (see section
2.7) on the current JNCC Conservation Importance List or NGO Birds of
Conservation Concern List. |
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- Lapwing:
This species was originally included on the conservation listings because the
UK holds greater than 20% of Europe’s wintering population.
Although the CBC does not monitor Lapwing strongholds in the north and
west of the UK, its substantial population decline in lowland England is of conservation
concern, especially when information from periodic national surveys is taken into
account (see Lapwing
Survey; Wilson et al. 2001).
- Cuckoo:
This species has declined more rapidly on woodland plots (-60%) than on farmland
CBC plots (-24%). The reasons for this
decline have not been investigated but may be linked to declines in the populations
of two key host species: Dunnock and Meadow Pipit.
- Yellow Wagtail:
The CBC index indicates that Yellow Wagtail numbers have decreased steadily since
the early 1980s. The trend suggested by WBS data is of even greater concern, indicating
an 84% decline in the size of the population between 1975 and 1999, although this
sample may not be representative of the population as a whole. Reductions in the
area of pasture in the UK and drainage of farmland may have driven this decline
(Gibbons et al. 1993).
- Mistle
Thrush: this is the third Turdus
thrush species to have declined sufficiently to rapidly trigger an alert. The
declines of these widespread and closely related species are of considerable conservation
concern. Research on Song Thrushes (Thomson
et al. 1997) and Blackbirds
(Siriwardena et al. 1998a)
suggests that decreasing survival rates have led to these declines and that this
mechanism may also apply to Mistle Thrushes too. The decline of the Mistle Thrush has been greater
on farmland CBC plots (-54%) than in woodland (-15%).
- Willow
Warbler: Detailed analysis of population data, survival rates and breeding
performance indicated that the population decline during the mid-1990s was largely
related to a fall in survival rates of adult Willow Warblers in the southern part
of the species' range in the UK (Peach
et al. 1995).
The decline is greater on woodland CBC plots (-52%) than on farmland plots (-19%).
- Reed
Bunting: This species is on the 50% conservation listing, but over the 31-year
period the magnitude of the population decline falls just short of the 50% mark.
The decline of Reed Buntings has been greater on WBS plots (-68%) than
on farmland CBC plots (-41%)
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| Table
4.2.2 >25% population alerts for CBC all habitats 1968-1999 |
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|
See Help for information on category definitions.
|
|
| Red-legged
Partridge, Curlew, Little Owl, House Martin, Meadow Pipit and House Sparrow are
excluded from this table as, although CBC data indicate the population sizes of
these species have all decreased by >25% during the last 31 years, in no case
was the observed decline statistically significant. |
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| 4.2.3 |
No
longer triggering alerts |
| Three species
are candidates for removal from the current JNCC Conservation Importance List
or NGO Birds of Conservation Concern List as they exhibit population declines
of <25% over the past 31 years: Kestrel (-4%), Swallow (+21%)
and Goldfinch (+18%). |
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| Although the
Kestrel does not trigger an Alert over the 31-year period, it does trigger a 25%
alert over the 25-year period. During the first few years of the CBC, the
data indicate that the size of the population was increasing from a relatively
low point, possibly reflecting a recovery from the detrimental effects of organochlorine
pesticide poisoning. This population decline
over 25 years is still a concern, given the species’ position at the top of one
of the open-farmland food-chains. |
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| Swallow and
Goldfinch have both now recovered from their population declines, which may have
been a consequence of medium-term fluctuations, perhaps driven by climatic events
or other factors. Alternatively, as previous reports used a less
sophisticated method of analysis than that which is currently employed, it is
possible that they may have indicated a decline mistakenly. |
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| 4.2.4 |
Alerts in farmland and woodland |
| In general,
more species raise alerts on farmland plots (20 species) than on woodland plots
(12 species) (see Appendix). Four species have declined sufficiently rapidly
to trigger alerts in farmland areas, but not over all CBC plots. Moorhens
have declined by 32% on farmland CBC plots since 1968, possibly due to a reduction
in number of farm ponds. Snipe have experienced a 64% drop in numbers in
lowland areas over the same period, a decline that may have been driven by reductions
in breeding success due to the drainage of wetland areas in order to provide agricultural
land. Land drainage may also be implicated in the 77% decline exhibited by the
UK Grasshopper Warbler population since the late 1960s. Farmland Blackbird
populations have decreased in size by 38% during this period, possibly due to
factors related to agricultural intensification. |
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| For a number
of species, sufficient samples of plots are censused to allow the comparison of
trends on woodland and farmland habitats. For two species the rate of decline
has been similar in both habitats: Spotted Flycatcher (farm -80%, wood
-81%) and Turtle Dove (farm -80%, wood -73%).
Both species are Palaearctic-African migrants and it is likely that the
declines have been driven by factors acting outside of Britain. |
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| For some species,
the declines experienced in farmland habitats have been greater than those experienced
in woodland habitats: |
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- Song Thrush (farm –69%;
wood –46%);
- Bullfinch (farm –65%; wood
–39%);
- Mistle Thrush (farm –54%;
wood -15%);
- Blackbird (farm –38%, wood
-9%).
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| For other species,
the declines have been greater in woodland than farmland: |
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- Starling (farm –64%; wood
–90%);
- Linnet (farm –47%; wood
–87%);
- Whitethroat (farm –37%;
wood –83%);
- Yellowhammer (farm –39%;
wood –76%);
- Dunnock (farm –38%; wood
–56%);
- Willow Warbler (farm -19%;
wood –52%);
- Cuckoo (farm -24%; wood
–60%).
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| For the most
part these are likely to reflect the habitat preferences of the species, with
declines being more rapid and populations recovering more slowly in the less preferred
habitat. |
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