Review of status and categorisation of feral birds on the British List Keith Vinicombe, John Marchant and Alan Knox, on behalf of the British Ornithologists' Union Records Committee he principal task of the BOU Records Committee is to maintain the list Tof species and subspecies which have occurred in Britain and Ireland. This work involves not only the assessment of records of birds potentially new to Britain, but also monitoring the status of all species to ensure that categori­ sations remain appropriate. Categorisation was introduced in 1971 to indicate those species for which there were no recent records (category B) and those whose breeding populations had originally been introduced, accidentally or on purpose, by man (category C). Species which have been recorded in an appar­ ently wild state at least once since 1st January 1958 arc placed in category A, which forms the bulk of the British and Irish List (BOU 1971, 1992a). Categorisation has brought its own difficulties in that the allocation of species to one category or another sometimes relics on fragmentary evidence and may depend on the subjective views of Committee members. For some species, more than one category is appropriate. One group which presents problems of categorisation comprises those species which occur fcrally. In the fifth and sixth editions of the BOU's check­ list of the birds of Britain and Ireland (BOU 1971, 1992b), four species were judged to have fallen into two categories and were given dual status. Three of these (Mute Swan Cygnus olor, Greylag Goose Anser wiser and Canada Goose Branta canadensis) were categorised AC because both wild and introduced birds occur or form the basis of our breeding populations. Capcrcaillie Tetrao urogal- his was categorised BC because the current introduced population replaced a wild one which died out in the eighteenth century. This paper discusses the results of a review by the BOURC of the status and categorisation of feral species in Britain (BOU 1993). The origins of the [Bril. Birds 86: 605-614, December 1993] 605 606 Status and categorisation of feral birds review lay in Committee discussions of the sixth checklist (BOU 1992b), fuelled by new information particularly from two important surveys: the Wild­ fowl & Wetlands Trust's 'Introduced Goose Survey' (IGS) in the summer of 1991 (Delany 1993) and the British Trust for Ornithology's New Atlas of Breed­ ing Birds in Britain and Ireland: 1988-1991 (Gibbons et al. 1993, referred to below as the 1988-91 Atlas). The review has led to the dualling of categories for several further species, to indicate cither AC or AD4 status, and the addition of others for the first time to category D4. The latter, a subgroup of category D, consists of species that would otherwise appear in Category C except that their feral populations may or may not be self-supporting. Category D4, like the rest of category D, is very much a 'wait-and-see' holding category and docs not form part of the main British and Irish List. During the course of the review, the Committee considered what consti­ tuted a viable self-supporting feral population, fulfilling the qualification for category C. It was agreed that rigid rules should not be applied. There was clearly a difference between, for example, one concentrated breeding popula­ tion of 900 Egyptian Geese Abpochen aegpptiams in East Anglia and a widely scattered population of 900 Barnacle Geese Branta leucopsis with sporadic and isolated breeding attempts. Also, the required population size would be differ­ ent for different species: 160 scattered Snow Geese Anser caerulescens may not be sell-sustaining, whereas 20 pairs of White-tailed Eagles Haliaeetm albicilla in Scotland could be. The populations of many feral species are dynamic: most of the scarcer ones can be expected to increase and perhaps qualify for higher categories in future years. In several cases, recategorisation arose from neither a change in status nor new information, but from new views within the Committee about how the categories should be implemented. For example, there have been no changes for decades in the status of feral Mallards Anas pkityrhymhos and Rock Doves (kdumha livia in Britain. Rather, it is the opinion of the Committee that these and other substantial feral populations deserve recognition and should not be ignored in faunal listings, whether at national or local levels. Species recategorised Pink-footed Goose Anser bmchyrhynchus The IGS (Delany 1993) found 88 at 29 sites. Some of these may have been non-returning winter- visitors. One pair in Lancashire had seven young. The 1988-91 Alias recorded breeding in two 10-km squares and summering in an additional 14. The Rare Breeding Birds Panel recorded two summer records in the UK in 1990 (Spencer et al. 1993). Clearly, this is a species not yet estab­ lished and not qualifying for category 0, but the Committee decided that it marginally justified dual AIM status to encourage further monitoring. Snow Goose A. caemltscem The IGS (bund 182 Snow Geese at 27 sites and at least 12% of those aged were juveniles. Since that survey, the Wildfowl ik Wetlands Trust has grounded 22 free-flying Snow Geese at Slim- bridge, Gloucestershire (Dr M. Owen in litl.}. In 1992, tire British feral population was estimated at 252, with 60 on Mull and Coll, Strathclyde (Evans 1993a). Feral breeding has been reported in a number of places and the population may be self-sustain­ ing (S. Delany in litl., Dr M. A. Ogilvie in litl). The 1988-91 Atlas, however, recorded breeding in only one square and summering in a further ten. The Committee considered that the population may at present be too widely scattered to be Status and categorisation offered birds 607 regarded as fully self-sustaining, and the species was given dual AIM rather than AC status. As with other category assignments, this will be reviewed from time to time. Bar-headed Goose A. indicus The IGS discovered 85, the highest counts being of 19 mine young) at Slraliield Save in Hamp­ shire and 11 at Highiieid I^ake, South Yorkshire. The 1988-91 Atlas records breeding in one square and summering in three more. A pair produced two young in Greater Manchester in 1992 (Mrs A.J. Smith in till). Clearly, the species is not established, but numbers are likely to increase, so the species was added to category D4 to encourage monitoring. Barnacle Goose Branla kumpsis The IGS found a total of 925 summering at 89 sites scattered throughout Britain. The largest flock was of 180 privately owned individuals at Seaview Marsh, Isle of Wight. Oflhose aged, 9% were juveniles, but, if the privately owned flocks are excluded (see below), this ligure rises to 17%. Breeding was noted at 15 sites. This figure of 925 is on a par with the 906 Kgyptian Geese found, and that species has, of course, been in category C since 1971. Barnacle Goose, however, differs in that the population is more widely scattered and 238 were based on three Wildfowl & Wet­ lands Trust reserves, at Slimbridgc, Gloucestershire, Washington, Co. Durham, and Arundel, West Sussex. If these and the Isle of Wight flock are taken out of the total, then the 'feral' popula­ tion becomes 507. The Wiklfowi & Wetlands Trust is aetivelv diminishing its stocks of Barnacle Geese by reducing their breeding success, in some years to nil (Dr M, Oven in till). The 1988-91 Atlas found breeding Barnacle Geese in 17 10-km squares from Hampshire to Northern Ireland and Jura, Strathelyde, in comparison with just two squares in the 1968-72 Atlas (Sharrock 1976). The 1988-91 Atlas notes that this species is 'probably still too scattered for there to be a self-sustaining feral population. If the current increase in breeding records continues, how­ ever, there seems to be no reason to doubt that such a population could become established/ It is also considered that the 1988-91 Atlas map under-represents the range in Britain. Simon Dclany {in lilt.) considers that the Barnacle Goose population is now probably Viable and self-supporting1. Clearly, the species is at least borderline. The Committee decided to exercise caution and con­ sidered thai the population may currently be too scattered to warrant dual category AC status, but there can be little doubt that, unless control measures are taken, the population will continue to increase and it will not be long before AC status may be justified. The species is, therefore, given dual AD4 status, but the position will be reviewed periodically. Muscovy Duck Cairma meschata A feral population of Muscovy Ducks has been present on the River Ouse at PJy, Cam­ bridgeshire, since the early 1980s (1988-91 Atlas). Broods were first noticed in 1987, and in November 1991 the population was estimated at approximately 130 individuals, of which about 60% were thai year's juveniles. The 1988-91 Atlas recorded breeding in four 10-km squares in Britain and summering in one other. The Committee considered that the Cambridgeshire popula­ tion is too recent to be added to category C, but this species obviously should be monitored. The species was therelbre added lo category D4. Gadwall Anas strepem Fox (1988) confirmed that 'there is no doubt that the present Gadwall breeding population in Britain derives largely from introduced stock'. The breeding population has been estimated at 770 pairs in Britain and 30 in Ireland (Gibbons el al. 1993). The winter population reached 7,700 in 1991/92 (Cranswick A al. 1992). In view of the size of this population, dual AC status is clearly overdue. Mallard A. plalyrkynckas The 1971 BOU checklist acknowledged that 'artificial rearing and re-stocking is widely practised' (BOU 1971).