Monitoring and conserving Scotland's birds of prey
In the second half of the 20th century barn owl numbers in England declined by more than half, due to changes in agricultural practices, a reduced availability of traditional buildings for nesting, and the introduction of pesticides for the control of rodents. Scottish birds must have suffered similarly, although the decline is less well documented as the early survey work did not extend north of the border, but here the distribution is also strongly influenced by climate.
Essentially a sub-tropical species with limited cold adaptation, the barn owl is at the northern limit of its global range in Scotland. It is widespread in the milder south and west, though at moderate elevations, becoming less numerous in the cooler north and east where most birds are confined to the coastal plain, and is known only as a vagrant to the Northern Isles. From the mid-1990s, a run of mild winters encouraged a range expansion that was most marked along the northern edge, and by 2000 breeding was established in both Caithness and Sutherland. However, these outposts will have been vulnerable to the cold winters of 2010 and 2011, which severely reduced the upland population in the South-west.
The stackyard, with its infestations of mice and rats, has been consigned to agricultural history, and the barn owl's role as "the farmer's friend" has less relevance now. Rough grassland, rather than arable farmland, has become the most important foraging habitat, and the field vole Microtus agrestis the main prey species. In northern temperate regions field vole populations are cyclic, typically building up to a peak every 3-4 years. In upland grassland and forestry, barn owl breeding performance has become strongly linked to the vole cycle and is characterised by large variations in annual productivity. Population turnover is rapid, with large pulses of recruitment interspersed with years when there are few yearlings in the breeding population. Barn owls breeding on lowland mixed farmland, with access to a wider range of small mammal prey, produce more modest broods on a regular basis.
Estimating the number of barn owls is notoriously difficult; there is little alternative to cold-searching for potential nest sites, and intensive local studies almost invariably discover a higher density than initially was suspected. The most recent estimate for the UK (1995-97) was c. 4,000 pairs, of which c. 550 pairs were in Scotland. The increase from the mid-1990s pushed the population closer to 1,000 pairs (Birds in Scotland, 2007). In 2007, 1432 nestlings were ringed in Scotland (per BTO) and the Scottish Raptor Study Group alone monitored 374 nests. There is little doubt that subsequent harsh winters have reversed this trend; the number of nestlings ringed in 2012 had fallen to 407. A clearer picture of the current distribution will become available with the publication of the New Breeding Atlas in autumn 2013.
While the long-term population trend is still uncertain, there are some positive signs. There is now a high level of awareness of the need for barn owl conservation in the countryside; habitat loss can be mitigated through agri-environment initiatives, and a great deal of effort has gone into the provision of artificial nest sites. Nest-boxes have been employed both to secure existing sites and to encourage the occupation of new areas where voles are plentiful. The most immediate threat in many parts of Scotland is the continued loss of traditional nest sites; old farm buildings are a non-renewable asset and there is an increasing dependency on nest-boxes, which will require current levels of provision to continue for the foreseeable future.
Shaw, G. and Riddle, G.S. (2003). Comparative responses of barn owls (Tyto alba) and kestrels (Falco tinnunculus) to vole cycles in South-West Scotland. In Thomson, D.B.A., Redpath, S.M., Fielding, A.H., Marquis, M., and Galbraith, C.A. (Eds.). Birds of Prey in a Changing Environment. The Stationery Office, Edinburgh. Pp. 131-136.
Shawyer, C.R. (1998). The Barn Owl. Arlequin Press. Shrewsbury.
Taylor, I. (1994). Barn Owls: Predator - Prey Relationships and Conservation. Cambridge University Press.
Toms, M.P., Crick, H.Q.P., and Shawyer, C.R. (2001). The status of breeding Barn Owls Tyto alba in the United Kingdom 1995-97. Bird Study 48: 22-37.
Hen Harrier• Goshawk• Sparrowhawk• Common Buzzard• Golden Eagle
Osprey• Kestrel• Merlin• Hobby• Peregrine Falcon• Barn Owl
Tawny Owl• Long-eared Owl• Short-eared Owl• Raven