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. Mirth America Whether ottered for sale Granivore Omnivore Insectivore

Advertised 9 5 0 Not advertised 2 4 20

3b. Asia Whether offered for sale Granivore Omnivore Insectivore

Advertised 4 3 2 Not advertised 1 1 6

Discussion To summarise these results, among the bird species listed in Appendix 1, North American granivores are significantly commoner in spring than in autumn. North American omnivores and insectivores, on the other hand, are more often recorded in autumn. Most of the North American granivore Rare passerines in Britain and Ireland 589 species that have occurred in Britain have been offered for sale here, just over half of ;hc omnivores have, but none of the insectivores has. The pattern among Asiatic species seems to be slightly different, but is less clear bxausc the numbers are smaller. Insectivores still predominate in the autumi , whereas granivores and omnivores are more often recorded in spring, tie latter significantly so. Advertising closely parallels the pattern for North / merican birds. In general, granivores and insectivores show the same pattern in the two regional data sets: the granivores are more likely to be advertised, and occur more often in spring; insectivores are rarely (if ever) advertised, and occur predominantly in autumn. Asiatic omnivores seem to resemble granivores in their more frequent occurrence in spring and witii a higher proportion being advertised. American omnivores predominate in the autumn, and just over half of the species have been offered for sale. While correlation is not causation, it is worth seeking an underlying reason for the apparent association between advertising and occurrence in the cases of insectivores and granivores. It seems reasonable to assume that most of the non-advertised species are of natural origin (although some may have escaped from captivity on die Continent, or been deliberately released because they were illegal—or difficult—to keep or sell). Their pattern of occurrence, with most (26 out of 34 species) occurring in autumn, fits well with expectations based upon our knowledge of bird migration (see references in Berthold 1991). Many species disperse at the end of the breeding season. Although this is usually relatively local, birds are very mobile, and it is perhaps not surprising that some individuals should stray hundreds or thousands of kilometres from their natal areas. Of those vagrants which have been aged, the majority are birds of the year undertaking their first migration. In the BBRC Reports for 1990 and 1991 (Rogers et al. 1991, 1992), 33 out of 34 and 34 out of 38 aged passerines recorded between August and October are listed as first-winter or juvenile. Experimental work has revealed that passerine night migrants—and this includes most of the species considered here—show age-dependent differences in migratory strategy (see Berthold 1991). Adult birds, having visited their wintering grounds at least once before, use complex navigational techniques, possibly also following learned landmarks, to complete their migration. Naive juveniles that migrate alone are supported only by an instinctive ability to fly in a particular direction. Their migration is thus more of a 'course and distance' procedure. Such inexperienced birds might easily overshoot the eastern seaboard of North America and be carried across the Adantic Ocean by fast-moving weather systems. The arrival of vagrants from Asia is less easy to explain. It has been suggested that, because of an error in physiology or behaviour, some individuals might orientate in the opposite direction to that in which they 'should' be going, leading them to northwest Europe rather than southeast Asia. Whether from North America or the east, it is unlikely that many of these vagrants successfully reorientate to winter in the 'correct' areas. Indeed, a plausible case may be made for spring occurrences, especially of non-advertised (presumably vagrant) species, being strays from the previous autumn that have wintered successfully in die Western Palearctic or Africa. 590 Rare passerines in Britain and Ireland If escape is independent of season (we have reasons for believing that it is not, but currently have no means of quantifying this), then approximately equal proportions would be expected in spring and autumn. It is striking that over half (13 out of 23, 57%) of the advertised species have occurred in spring, compared with one-third of the species that have not been advertised (8 out of 34, 34%), and this difference is statistically very significant5. While it is possible to construct explanations based upon differences in migratory patterns, it could be suggested that this is just the pattern that would be expected if the records of advertised species were all escapes. A case could be made for not accepting any records of species that have ever been held in captivity. This seems rather draconian, and would undoubtedly lead to the rejection of a number of genuine vagrants. The BOURC will, however, continue to look very closely at those species that turn up in Britain and are currently included in the upsurge in importations from China and the former USSR. The present practice of the BOURC of placing records of doubtful or uncertain origin into category D will continue, as this encourages the gathering of records and further information which may men be reviewed from time to time. Meanwhile, birders arc encouraged to submit detailed records of all exotics to their county recorders. Recorders arc urged to publish acceptable records in their local reports. Collation of these data on a national scale is already being investigated by the Association of County Recorders and Editors (ACRE). It would be extremely valuable to have a representative data set of obvious escapes, such as Canaries Serinus canaria, or African and Australian seed-eaters. Knowing the pattern of occurrence in time and space of birds that have certainly escaped or been released from captivity would gready facilitate the task of assessing the status of those that might have arrived here more or less naturally. It should be noted that this analysis does not take into account the bird trade in continental Europe, the commonness of a particular species in captivity or in the wild, migration strategics or a variety of other relevant factors. Nevertheless, the emerging patterns are quite clear: granivorous birds arc advertised for sale more often than are inscctivorcs, and occur more often in spring than in autumn. Non-advertised species (mainly insectivores and North American omnivores) appear far more often in autumn than in spring. The most tempting interpretation is that the rarest, long-distance, passerine vagrants occur naturally almost exclusively in the autumn, and that spring birds may be escapes. Such conclusions would, however, be premature, and further work is needed to examine alternative explanations.

Acknowledgments We arc extremely grateful to Tim Inskipp for making available the records of" advertising in Cage and Aviaiy Birds that he has so painstakingly collected for 20 years. We are also grateful to Dr Colin Hradshaw and to our colleagues on the British Ornithologists' Union Records Committee (Ian Dawson, Tim Inskipp, Peter Lansdown, John Marchant, Tony Marr, John Mather, Richard Porter, Ken Shaw and Keith Vinicombe) for commenting upon earlier versions of the manuscript. Rare passerines in Britain and Ireland 591 Summary Records of 57 long-distance vagrants and potential vagrants to Britain and Ireland from North America and Asia are analysed, and related to their diets, seasons of occurrence and whether they have been advertised for sale in Britain. Striking differences exist which suggest that many of the spring records should be viewed with caution. Birders are urged to submit, and editors to publish, all records of exotics (especially wildfowl, raptors and passerines) to assist with an assessment of seasonal and geographic patterns of occurrence.

References BK.RTHOLD, P. 1991. Orientation in birds. Experimtia Suppl. vol. 60. Basel. CHARLTON, T. I). In press. Lark Sparrow in Suffolk: new to the Western Palearctic. Brit. Birds. UIXKY, A. E., FERGUSON, A., HKYWOOD, R-, & TAYLOR, A. R. 1981. Aleutian Tern: new to the Western Palearctic. Brit. Birds 74: 411-416. DliRAND, A. L. 1963. A remarkable fall of American land-birds on the 'Mauretania', New York to Southampton, October 1962. Brit. Birds 56: 157-164. DYMOND, J. N., FRASKR, P. A., & GANTLK'IT, S. J. M. 1989. Rare Birds in Britain and Ireland. Calton. RoGKRS, M. J., & THF. RARITIES COMMITTEE. 1991. Report on rare birds in Great Britain in 1990. Brit. Birds 84: 449-505. 1992. Report on rare birds in Great Britain in 1991. Brit. Birds 85: 507-554. HARRISON, C. 1982. An Atlas of the Birds of the Western Palaearctk. Ijondon. MCKAY, C. R. 1994. Brown-headed Cowbird in Strathclyde: new to Britain and Ireland. Brit. Birds 87: 284-288. SoKAI, R. R., & Rom J', G.J. 1981. Biometry (2nd edn). Freeman, San Francisco. WAI.DON, J. 1994. Ancient Murrelet in Devon: new to the Western Palearctic. Brit. Birds 87: 307- 310. WKI.LS, H., & KING, J. F. 1981. A general 'exact test' for N X M contingency tables. Bull. Southern California Acad. Sa. 79: 65-77.

Dr David T. Parkin, Department of Genetics, Queen's Medical Centre, Nottingham NG7 2UH Dr Alan G. Knox, Buckinghamshire Count): Museum, Tring Road, Halton, Buckinghamshire HP22 SPJ Appendix 1. Species included in analysis Also shown are the diets (Ins = insectivore, Omn = omnivore, Gm = granivore), region of origin (North America or Asia), season of occurrence (spring or autumn) and whether advertised for sale in Cage and Aviary Birds (Yes = advertised)

Species Diet Origin Season Advertised

Kastcrn Phoebe Sayornis phoebe Ins NAm Spr — 'Free Swallow Tachycineta bkolor Ins N Am Spr — Cliff Swallow Himndo pyrrhonota Ins NAm Aut — BlyuYs Pipit Anthus godlewskii Ins Asia Ant Cedar Waxwing Bombycilla cedrorum Omn NAm Spr Yes Brown Thrasher Toxoshma rufum Omn NAm Aut — Grey Catbird Dumetella carolinensis Omn NAm Aut White-throated Robin Irania gutturalis Omn Asia Spr — Daurian Redstart Phomicurus auroreus Omn Asia Spr Yes Varied Thrush ^pothera naevia Omn NAm Aut Yes Wood Thrush Hylocuhhi mustdina Omn NAm Aut Grey-cheeked Thrush Catharus minimus Omn NAm Aut — Veery C. juscescens Omn NAm Aut Yes Pallas's Grasshopper Warbler Ijocustelhi certhiola Ins Asia Aut - Thick-billed Warbler Acrocephalus aedon Ins Asia Aut — Pallas's I .eaf Warbler Phylhswpus proregulus Ins Asia Aut — 592 Rare passerines in Britain and Ireland

Species (eontinucd) Diet Origin Season Advertisf

Raddc's Warbler P. schivarzi Ins Asia Aut — Asian Brown Flycatcher Mvscicapa dauurica Ins Asia Spr — Mugimaki Flycatcher Ficedula mugimaki Ins Asia Aut Yes Brown Shrike Ixinius tmlatus Ins Asia Aut Yes Daurian Starling Stumus stuminus Omn Asia Spr Yes White-checked Starling S. cineracms ()raii Asia Spr Yes Yellow-throalcd Vireo Vireo flamfttms Ins N Am Aut — Philadelphia Vireo V. philadelphicm Ins NAm Aut — Red-eyed Vireo V. olwaceus Ins N Am Aut — Red-fronted Serin Smnus pusiUus Grn Asia Spr Yes Pallas's Roselinch Carpodaais mseus Grit Asia Spr Yes Evening Grosbeak Hesperiphona vespertma Grn NAm Spr Yes Tennessee Warbler Vmnwara peregrma Ins NAm Aut — Northern Parnla Parula americana Ins NAm Aut — Yellow Warbler Dmdroka petechia Ins NAm Aut — Chestnut-sided Warbler D. pemybanica Ins NAm Aut — Blackburnian Warbler D. ftisca Ins N Am Aut — Cape May Warbler D. tignna Ins NAm Spr — Magnolia Warbler D. magnolia Ins NAm Aut — Palm Warbler D. palmanun Ins NAm Spr — Blackpoll Warbler D. striata Ins NAm Aut — •American Redstart Setophaga mtirilla Ins N Am Aut — Ovcnbird Seiums- aurocapilhs Ins NAm Aut — Northern Waterthrush S. noveboracemrs Ins NAm Aut — Hooded Warbler Wikonia cilrina Ins NAm Aut — Wilson's Warbler W. pusilh Ins NAm Aut — Summer Tanager Piranga rubra Omn NAm Aut Yes Scarlet Tanager P. ulivacea Omn NAm Aut Yes Rufous-sided Towhee Pipih erylkmphthalmus Grn NAm Spr Yes Lark Sparrow Chondestes grammacus Grn NAm Spr — Fox Sparrow' ^pnotriehia iliaca Grn NAm Spr Yes Song Sparrow £ melodia Grn NAm Spr — White-crowned Sparrow £ imcapkrys Grn NAm Spr Yes Yellow-browed Bunting Emberiza chrysophrys Grn Asia Aut Yes Chestnut Bunting E. rutila Grn Asia Spr Yes Indigo Bunting P&ssmna cyanea Grn N Am Aut Yes I aznli Bunting P. amnena Grn N Am Aut Yes Painted Bunting P. am Grn NAm Spr Yes Bobolink Dolichonyx oryzivoms Grn N.Am Aut Yes Brown-headed Cowbird Muhthrm ater Grn NAm Spr Yes

Appendix 2. Statistical tests and results 1. Table 1 a, heterogeneity estimated as x' = 11.4, dX = 2, p < 0.01. Table lb, heterogeneity estimated as x' = 26.4, d.f. = 2, p < 0.01. 2. Heterogeneity among tables estimated using a log-linear model for the analysis of multi­ dimensional contingency tables (Sokal & Rohlf 1981). Estimate of heterogeneity given as G = 12.4, d.f. = 2, jO < 0.01. 3. Using a modification of Fisher's exact test (Wells & King 1981). 4. p < 0.02 in both cases 5. p < 0.01 6. p < 0.02 7. p < 0.37 8. p < 0.01