The Rook - a successful farmland
bird
Malcolm Ogilvie
The
Rook is one of our most familiar and widespread birds. With nearly
one million pairs in Britain, it is only absent from upland areas
and from treeless islands off the west coast of Scotland, though
it has been able to find suitable trees to nest on Shetland and
on Lewis in the Western Isles.
Rookeries are
among the most conspicuous of colonial nesting sites, not least
because breeding generally starts before any leaves have appeared
on the trees. In Scotland, though, the commonest tree used, holding
about 50% of all nests, is the Scots pine and so the nests are not
nearly as visible as in broad-leaved trees, of which ash, beech,
elm, oak and sycamore are the most regularly used. When I lived
in Gloucestershire, I took part in a long-running annual survey
of rookeries in the Severn Vale, an area dominated by elm trees,
planted as windbreaks and growing out of the many hedgerows. They
held the majority (about 85%) of the rookeries, but when Dutch elm
disease wiped out over 99% of the elms in the early 1970s, we feared
that their loss might lead to a severe reduction in Rooks, but we
had underestimated their adaptability and resourcefulness as they
shifted to other species, notably ash and sycamore, and as a result
numbers hardly dropped at all
Although locating
rookeries rarely presents any problems, actually counting their
nests with any degree of accuracy is not all that simple because
the bulky twig nests, placed among the smaller branches and twigs
near the tree tops, often join on to each other. When one is standing
on the ground and craning one's neck upwards it can be quite difficult
determining whether a particularly large assemblage of twigs is
the product of just one particularly industrious pair, or an amalgamation
of several nests.
Rook
numbers in Britain were first estimated in the mid-1940s when the
BTO carried out a survey at the request of the Agricultural Research
Council. It didn't cover the whole country, but the extrapolated
total was around three million birds. Although numbers probably
increased thereafter, they fell sharply in the 1960s and early 1970s
to no more than two million (or 900,000 breeding pairs) by the time
of the next census, in 1975. Agricultural pesticides were thought
to be the main reason for the decline. This view has been reinforced
by a recent recovery to 1.27 million pairs in 1996, a 40% increase
over 1975 but still below the level of the mid-1940s.
Unable to break
the habit of counting rook nests when I moved here from Gloucestershire,
each spring I go out and survey Islay's ten or eleven rookeries,
which together hold a little over 300 nests. Even in a short span
of years I have detected some changes, numbers declining in one
area or building up in another. There was also the spring when I
went to check on one of our largest rookeries, holding over 60 nests,
to find it completely deserted and the Rooks busy building in another
wood half a mile away. The reason for the desertion wasn't difficult
to find: a pair of Buzzards had decided that the old rookery was
an ideal place for them to nest!
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Dr
Ogilvie is a natural history writer and editor, formerly a research
scientist with the Wildfowl and Wetlands Trust, and resident
on the island of Islay since 1986. Until 1997, a member of the
'British Birds' editorial board and also one of the editorial
team which produced 'Birds of the Western Palearctic'. |
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