The Decline of the Raven, Corvus corax, in Relation to Afforestation in Southern Scotland and Northern England

Abstract

The breeding raven population of southern Scotland and Northumberland [England] was formerly (pre-1960) characterized by year-to-year stability in both numbers and distribution. The 123 known pairs occupied traditional nest-sites on cliffs or trees. Breeding density was correlated with altitude (greatest on high ground) and land productivity (greater on base-poor granite than on sedimentary rocks). This was probably linked with a greater availability of sheep carrion on high ground and base-poor ground. The breeding population has been declining since the 1960s, and in 1974-1975 only 55% of former regular nesting areas were still occupied, with breeding pairs reduced to 44%. This was mainly associated with the afforestation of former sheepwalk. The argument is based on a geographical parallel (greatest decline in areas with most afforestation) and on a temporal parallel (desertion of particular nesting areas coincident with planting in the area surrounding the nest sites). The level of afforestation at which ravens deserted varied between nesting areas. Not all desertion of nesting areas could be attributed to afforestation: 4 raven pairs were dispossessed from cliff nest-sites by golden eagles which recolonized southern Scotland over the period considered, and at least 5 other pairs by rock climbers. Sheep management also improved over the years, and may have contributed to the desertion of some marginal nesting areas, by leading to a reduction in the amount of carrion available. In the Lake District, a comparable hill area with practically no afforestation, there was no decline in the numbers of breeding ravens over the same period. Among the pairs studied in 1974-1976, those occupying the most heavily afforested ground showed more non-breeding, and produced later, smaller broods than did those on less afforested ground. Those nesting areas still occupied and where more than 25% of former sheepwalk within 5 km had been afforested, produced later and smaller broods than they did before afforestation. To judge from pellets, sheep carrion formed a major part of the diet, but a great variety of other foods available from grassland was also eaten. The percentage frequency of sheep remains in pellets declined significantly with increasing afforestation in the area around the nest-sites. The prediction is made of a further decline in raven breeding numbers if blanket afforestation continues to expand over former upland sheepwalk.