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Habitat
changes in British upland landscapes
and their implications for bird populations
by Rob Fuller |
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2. Habitat changes in
the uplands associated with farming |
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| Two main agricultural processes
have been especially significant for birds in the British uplands.
Firstly, the widespread improvement of grassland at the hill edge
and, secondly, changes in livestock systems, in particular large increases
in numbers of sheep. |
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2.1
Grassland and moorland improvement |
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The term 'grassland improvement' embraces
a complex of farming activities aimed at raising land productivity;
these include drainage, ploughing, fertilising, re-seeding and
herbiciding. This has often been a precursor to higher stocking
and / or a switch from hay to silage. There is considerable
evidence that numbers of ground-nesting birds have declined
in moorland-edge habitats (Baines 1988, 1989, 1990, Fuller et
al. 2002). |
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Numbers
of breeding curlews and other waders have declined in marginal
upland habitats as a result of changes in grassland management. |
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| These include populations of breeding waders - notably
Lapwing, Redshank, Snipe and Curlew - and of passerines including
Skylark, Twite and Reed Bunting. Loss of damp and rough grassland
appears also to be a factor in local declines of Black Grouse and
Yellow Wagtail (Ratcliffe 1990). There has been widespread improvement
of enclosed land close to open moorland in many upland regions of
Britain and these declines in ground-nesting birds are probably associated
with these habitat changes. Vickery et al. (2001) describe
mechanisms by which changes in lowland grassland management have affected
birds and their food resources; these are broadly relevant to upland
grasslands. Deterioration of habitat quality through loss of damp
feeding areas is probably a key mechanism for breeding waders. Changes
in the structure of swards have also been important for many birds.
Where improvement has resulted in relatively simple, uniform swards,
this can result in increased predation pressure on nests and chicks.
Where there has been a switch to silage crops, the vegetation may
be too dense for birds to nest and forage efficiently. A general effect
of grassland improvement has been reduction of abundance of seeds
which is especially important to buntings and finches. |
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| Under some circumstances,
grassland and moorland improvement may cause a local increase
in some species. Oystercatchers for example, frequently appear
to benefit from grassland improvement (Ratcliffe 1990). In the
Uists, Outer Hebrides, patches of moorland were fertilised and
reseeded in the 1980s. Some of these green 'reseeds' have been
colonised by Oystercatcher, Lapwing, Redshank and even Dunlin
(Pienkowski et al. 1986). These reseeds may, however,
be unusual in having substantial wet areas remaining within
them. Where drainage occurs on open moorland, it is likely that
high densities of breeding waders would not be sustained. Furthermore,
other species of moorland birds that are less obviously birds
of wet places, such as Red Grouse and Meadow Pipit, may depend
on flushes as sources of insect food for their chicks (Hudson
1988). |
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| Numbers
of territories of four breeding bird species on two farms
in marginal uplands between 1976 and 1988. Solid lines are
numbers on Farm 1 (Lancashire) and dotted lines from Farm
2 (West Yorkshire). From Fuller et al. (2002). |
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In some areas, especially
in areas of Welsh ffridd, and in parts of Scotland, land improvement
in the marginal uplands has involved scrub clearance through
cutting or burning. Gorse scrub is especially vulnerable.This
can cause local habitat loss for many relatively common scrub-nesting
birds. |
Reed Buntings
have fared badly on marginal upland farms in England. |
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2.2.
Grazing pressure |
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| Grazing pressure has increased in many
upland areas as a result of increases in numbers of sheep or deer.
Large increases in numbers of sheep have occurred throughout the main
sheep-farming regions of Britain since the mid 1970s with numbers
tending to stabilise or slightly decline since around 1990 (Fuller
& Gough 1999). The upland regions that have undergone the largest
increases in sheep are Wales and northern England. Within Scotland,
the greatest increases in sheep have generally been evident in the
Southern Uplands but locally there have been large increases in stocking
in several parts of Scotland. |
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The improvement of grassland
described above often precedes an increase in sheep numbers
so in practice it can be difficult to distinguish the effects
of land improvement from those of grazing. Nevertheless, the
widespread increase in grazing pressure by sheep over the last
30 years has probably been generally deleterious to bird communities
in upland regions (Fuller 1996, Fuller & Gough 1999). In
general the species that have benefited either tend to be ones
that use very short swards, notably Wheatear, and scavengers
or generalists, notably the corvids. |
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The
number of sheep has increased in most areas of Britain in
recent decades. Numbers in Wales have increased steadily since
1950, but many other regions have shown increases since the
1970s. Although numbers have tended to decrease since the
1990s sheep still remain far more numerous than in the mid
1970s. |
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| Intensified grazing by sheep has affected
upland birds through three main mechanisms. Firstly, grazing can alter
vegetation types that are preferred by some species. The loss and
fragmentation of heather is a principal concern in this respect but
alteration of vertical and horizontal vegetation structure is probably
also important. Spatial complexity of sward structure in terms of
patchiness, height and foliage density appears to be important to
several bird species in providing cover for nests and chicks, and
feeding sites. Heavy grazing tends to reduce such sward complexity.
Secondly, heavy grazing can have complex effects on the food resources
used by birds, such as fruit, invertebrates, small mammals and carrion. |
| Heavy grazing
is likely to actually increase availability of soil and dung-associated
invertebrates for Starlings, Magpies, Jackdaws and Rooks. Set
against this, however, the widespread use of avermectins may
act to diminish populations of dung invertebrates. Heavy grazing
generally results in a reduction of the small mammals upon which
several owls and Kestrels depend. |
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Increasing
numbers of deer have caused added grazing pressure in the
uplands. |
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| Carrion is a key resource
for Ravens, Crows, several birds of prey and foxes. In many areas
the increase in sheep numbers since the 1970s probably led to larger
amounts of carrion for scavenging birds. However, there are no simple
general relationships between numbers of sheep and carrion because
husbandry and stock management practices have to be taken into account
as well as sheep density. Thirdly, heavy grazing may affect predation
of ground-nesting birds in various ways. By altering the structure
and pattern of vegetation, grazing could influence the probability
of nests being detected by predators. It may also enhance some of
the other food resources used by predators, notably carrion and soil
invertebrates. This could lead to increases in numbers of corvids
but whether this would necessarily result in an increase in predation
pressure on ground-nesting birds is uncertain.
Grazing by deer is not specifically an agricultural issue but it
is appropriate to mention it here because the mechanisms by which
moorland birds are affected are very similar to those involved with
sheep grazing. |
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The issue
of deer grazing in the uplands is principally a Scottish one
and mainly concerns Red Deer, which approximately doubled in
numbers between the 1950s and 1990s. One of the main concerns
about deer numbers is their effect on woodland habitats, particularly
the native pinewoods. Suppression of the field layer appears
to be a contributory factor in the decline of Black Grouse and
Capercaillie (Baines 1996, Baines & Hudson 1995, Baines
et al. 1994). There is also a wider issue of the impact
that deer may have on natural regeneration of trees, both within
and outside existing woodland. |
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Heavy
grazing of the field layer appears to be a contributory factor
in the decline in birds such as the Black Grouse. |
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