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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