Occurrence patterns of rare passerines in Britain and Ireland David T. Parkin and Alan G. Knox,, on behalf of the British Ornithologists' Union Records Committee n recent decades, there have been increasing numbers of records in Britain Iand Ireland of birds that have occurred vast distances outside their normal ranges. It is the difficult task of the British Ornithologists' Union Records Committee (BOURC) and die Irish Rare Birds Committee to decide which of these have occurred naturally, rather dian as a result of escape from captivity. The species involved may be divided into four groups: 1. Species that arc unlikely candidates for vagrancy to Britain and Ireland, that are not held in captivity, and whose passage is unlikely to have been ship- assisted (e.g. Ancient Murrelet Synthliboramphus antiquus and Aleutian Tern Sterna aleutica: Waldon 1994; Dixey et at. 1981). These species presumably arrived in Britain quite naturally, unless there are people who, for their own motives and quite illegally, are importing exotic species for subsequent release. 2. Species that are unlikely candidates for vagrancy to Britain and Ireland, that are not commonly held in captivity, and whose arrival may have been ship- assisted (e.g. Lark Sparrow Chondestes grammacus and Brown-headed Cowbird Mokthrus ater. Charlton in press; McKay 1994). Some of diese have occurred close to seaports having regular trade with relevant parts of the world, and some individuals were even see to fly ofF ships close to land or shortly after docking (e.g. Northern Flicker Cokptes auratus: Durand 1963). Unless mere is direct evidence of human assistance, as with the Snowy Sheathbill Ononis alia brought to Plymouth from the Falkland Islands (Brit. Birds 75: 591), such species are currently eligible for admittance to Category A of the British & Irish List. Individuals known to have benefited from active human assistance (having been fed or sheltered) are excluded from Category A, but may be placed in Category D, which does not form part of the main list. 3. Species mat are very unlikely or impossible natural vagrants to Britain and Ireland, and which are commonly kept in captivity in Britain or neighbouring countries (e.g. Budgerigar Melopsittacus undulatus and Chiloe Wigeon Anas \ISnl. Hmlsi',1- 5&ri-5!>2, IVcrmlx-r 1«94| 585 586 Rare passerines in Britain and Ireland sibikitrix). Such species are not admitted to the British & Irish List, unless it is proven that they now have feral breeding populations that are self-sustaining without recourse to further releases or escapes (e.g. Ruddy Duck Oxyura jamakensis and Golden Pheasant Chysolophus pictus), in which case they are placed in category C. which does form part of the main list. 4. Species tfiat arc possible natural vagrants to Britain and Ireland, although they are also kept in captivity and may have escaped or been released. This paper examines one aspect of the pattern of occurrence of this last group of potential rarities: the link with captivity. Statistical tests have been used to investigate differences in the pattern of occurrence with respect to season, normal range, diet and whether the species occurs in captivity in Britain. Methods For the purposes of this analysis, only passerine long-distance migrants have been considered. Of the main non-passerine groups, ducks and raptors have been omitted because of the frequency with which they occur in, and escape from, captivity. Waders and scabirds have been excluded since they arc scarce in collections. The species included are all among those for which a full description is required for acceptance by the British Birds Rarities Committee (BBRC), and the analysis has been further restricted to those species that can be considered as long-distance migrants and which breed in either North America or Asia. The distribution of the latter group has been taken from Harrison (1982). A total of 57 species is included in die analysis. The records used have been taken from Dymond ei al (1989) and subsequent reports (to October 1992) of the BBRC (published annually in British Birds) and the British Ornithologists' Union Records Committee (published in Ibis). The data include all accepted records of the selected species, all records where the identification was accepted by the BBRC but doubt existed about the natural vagrancy, and a few records under consideration by the BOURC. Records that have not been published by the BBRC, or records that have not yet reached the BOURC, have been excluded. To simplify the analysis, the species have been grouped into those which occurred in spring (the first sighting of each record was in April to July) or autumn (first seen in August to November), but not in both seasons. Species first reported in December to March have been considered to be wintering, and have been excluded. An indication of the species' presence in captivity has been obtained from advertisements in the weekly aviculturists' newspaper Cage and Aviary Birds (CAB). This information was provided by T. P. Inskipp (Secretary of the BOURC) who, for the past 20 years, has maintained extensive records of species offered for sale and their changing prices. He kindly advised which of the 57 species under consideration have been offered for sale or exchange at least once since 1970. It should be emphasised that this information takes no account of additional species known to be involved in the cage-bird trade in mainland Europe, and will probably exclude some species that are held in captivity in Britain and/or Ireland, but have never been offered for sale here. Rare passerines in Britain and Ireland 587 It is also possible that occasional relevant advertisements were overlooked. None of these factors should profoundly affect the results. The complete data arc presented in Appendix 1, where a comparison is made between species from North America and those from Asia, the season of occurrence in Britain and Ireland, and whether or not they have been advertised in CAB. Species such as Red-flanked Bluetail Tarsiger cyanurus have been omitted because they breed west of the Ural Mountains, and others, such as White's Thrush ^pothera dauma and Dusky Warbler Phylkscopus juscatus, because they have been recorded in spring or winter, as well as autumn. We have included a further variable in the analysis: diet. Certain species or families arc easier to keep in captivity, and hence arc commoner cage-birds. For example, sccd-eatcrs tend to be more popular than inscctivores. In Appendix 1, the diet of the species is also indicated as primarily insectivorous (e.g. flycatchers, warblers), granivorous (finches, buntings, sparrows) or mixed (starlings, thrushes, chats), here called 'omnivorous' for convenience. Results There arc clear differences in both advertising pattern and season of occurrence among species with different diets (tables la & lb)'. Of die species listed, a higher proportion of the granivores occurs in spring than is the case for either the omnivores or the inscctivores, both of which are commoner in autumn. Also, granivores are much more likely to be advertised than omnivores. Inseetivores are rarely offered for sale. Table 1. Occurrence patterns of rare passerines in Britain originating from North America and Asia Tabic la shows the number of each dietary class occurring in spring and autumn; table lb shows the number advertised and not advertised for sale in avicultural magazines. Data from Appendix 1. For further details, see text la. Both Regions Season of occurrence Granivore Omnivore Inscctivore Spring 11 5 5 Autumn 5 8 23 lb. Both Regions Whether offered for ;sal e Granivore Omnivore Inseclivore Advertised 13 8 2 Not advertised 3 5 26 Pooling the data from North America and from Asia like this does, however, mask a difference between them. The relevant figures for die two regions are shown separately according to season of occurrence (tabic 2) and advertising status (table 3). Comparison of the data for North America and for Asia (tables 2a & 2b) shows a significant difference between them2, so the data are heterogeneous and die regions must be analysed separately, despite the smallness of the sample sizes. All superscript numerals refer to Appendix 2, where statistical details are noted. 588 Rare passerines in Britain and Ireland Table 2. Data from table la partitioned by region of origin For further details, see text 2a. North America Season of occurrence Granivore Omnivore Insectivore Spring 8 1 4 Autumn 3 8 16 2b. Asia Season of occurrence Granivore Omnivore Insectivore Spring 3 4 1 Autumn 2 0 7 For each region (tables 2a & 2b), wc have tested to see whether the seasonal pattern of occurrence is the same in the three dietary groups.3 This reveals that die pattern is different for the two regions*. Looking at the North American species more closely, a higher proportion of granivores occurs in the spring than for either the omnivores5 or the insectivores6. The latter two groups do not differ in dieir pattern7. Among die Asian species, a significandy higher proportion of omnivores occurs in spring compared with the insectivores". The proportion for spring granivores does not differ significandy from that for either of the other two groups, although, like omnivores, diey occur more in spring. Turning to the pattern of advertising, we have repeated the above analysis3. For North American species (table 3a), the proportion of advertised birds differs among the three dietary groups5. It is striking mat no insectivores have been offered for sale, contrasting markedly with 82% of granivores and 55% of omnivores. Among the Asiatic species (table 3b), the frequency of advertising is almost identical for granivores and omnivores; as with the North American data, it is significantly lower for insectivores (25%). Table 3. Data from table lb partitioned by region of origin For further details, see text 3a.