Barn owl release in lowland southern England—a twenty-one year study

Abstract

Twenty-one years of carefully documented barn owl (Tyto alba) study and release in England by the South Midlands Barn Owl Conservation Group (SMBOCG) have necessitated a re-examination of some of the premises on which such release schemes are judged. Fifteen years after the first releases, the number of independently breeding owls (i.e. breeding ‘wild’ or ‘as though wild’) in a 1200 km² study area is still increasing sharply and now far exceeds the number released in any one year, with the rate of increase outstripping the cumulative number of release events. Ringing returns indicate that fledged young dispersed further than adults, and that released adults which immediately deserted their release sites were less likely to survive their first 30 days than those staying in the vicinity of their release, where they could take advantage of supplementary feeding. Artificial feeding at release sites led to pellets containing lower liveweight equivalent of wild-caught prey but higher numbers of fledged young. Otherwise, and importantly, no significant difference in survival, mortality, dispersal, foraging success or breeding success was found between wild/independent owls and various categories of released bird. Availability of field vole habitat was not as important for barn owl breeding success as was found in previous studies, with owls able to replace this species in their diet with the less habitat-specific wood mouse. Evidence is provided that starvation was not as important a cause of mortality as has been proved elsewhere. Although barn owl nests often occurred in extreme close proximity, breeding density had no effect on either foraging or breeding success. The possible genetic consequences of barn owl release in Britain are briefly discussed. Very few barn owl release schemes have attempted to prove that their work is worthwhile, so long-term, well-documented studies such as that presented here are vital in establishing some of the principles involved in releasing barn owls into the wild.

Introduction

Carefully considered reintroduction schemes are now part and parcel of the modern conservationist's tactical armoury (e.g. Hodder and Bullock, 1997). Several bird of prey species have been the subject of apparently successful introductions into Britain, including red kite Milvus milvus, white-tailed eagle Haliaetus albicilla, osprey Pandion haliaetus and goshawk Accipiter gentilis. In each case, donor birds for these introductions were taken from other European countries. Despite this, barn owl release schemes in Britain using British stock have been opposed by most leading conservation organisations. Concerns can be summarised as: (1) animal welfare (i.e. that released individuals are likely to starve or otherwise perish); (2) released individuals may harm remnant wild populations by introducing disease, or competing for limited habitat and food supply; (3) behavioural/physiological differences in captive birds may make integration with wild birds difficult, (4) there is no resulting population increase or conservation benefit, (5) the genetic consequences of releasing captive-bred barn owls (e.g. Rebane and Andrews, 1995). DEFRA has now issued a consultation paper with a mind to discontinuing the Captive Bred Barn Owl Release Scheme (DEFRA, 2001). Twenty-one years of carefully documented barn owl study and release in England by the South Midlands Barn Owl Conservation Group (SMBOCG) have necessitated a re-examination of some of the criteria on which such reintroduction schemes are judged.

Despite remaining a globally abundant species with a near-worldwide distribution, the barn owl (Tyto alba) experienced a steady decline in Britain during the twentieth century (Blaker, 1934, Prestt, 1965, Parslow, 1973, Sharrock, 1976, Shawyer, 1987, Percival, 1991). The causes of this decline are variously cited as: (1) loss of field vole habitat due to intensification of agriculture and expansion of towns; (2) loss of nest sites due to barn conversions or dutch elm disease; (3) severe winters with prolonged snow cover; (4) the disappearance of the corn rick (winter home of owls prey species) since the advent of the combine harvester; (5) chemical poisoning by organochlorine pesticides such as DDT, aldrin and dieldrin during the 1950s and 1960s (Newton et al., 1991); (6) increase in road traffic causing greater numbers of road casualties (Newton et al., 1997).

By 1985, the barn owl had become very scarce over large parts of the English midlands. According to Shawyer (1987), densities of barn owls in the counties of Derbyshire, Staffordshire, Leicestershire, Warwickshire, Northamptonshire, Buckinghamshire and Oxfordshire averaged only 1.6 pairs per 100 km² between 1982 and 1985, well below his estimated critical threshold for population sustainability of 2.5 pairs per 100 km². In an initial study by the SMBOCG of a 900 km² area contained within the counties of Oxfordshire, Northamptonshire and Warwickshire only five breeding attempts were recorded during the period 1979–1985 (with only two of those producing young to fledging), despite the examination of a large number of likely outbuildings and tree cavities in which barn owls could potentially roost or breed. Twenty-six nestboxes were provided by SMBOCG in the study area in 1983, but by 1985 none had been used for breeding, and only four for roosting. Recovery of barn owl numbers in this area by natural means did not seem likely, or at best would be extremely slow. Releases of barn owls in this area started in 1986.

Using a dataset compiled between 1979 and 2000, this paper aims to test whether it is possible to conduct a barn owl release programme such that (1) the foraging and breeding success, dispersal and mortality of released owls and their offspring do not differ from those of wild/independent owls, and (2) re-establishment of a viable and sustainable breeding population of barn owls in the wild can occur from initially low levels.