While this may seem an extraordinary question, it was one of the major agenda items during the 2004 BBRC Annual Business Meeting, held at Dungeness Bird Observatory, in Kent, in February. Debating our remit will help with some of the increasingly complex decisions we now have to make. These range from general issues such as how to manage our workload and how to assess rare subspecies in a time of constant change, through to specific questions such as what do we do with Pine Buntings Emberiza leucocephalus which show some minimal yellow tones in the outer webs of the primaries? These are examples of the changing world of record assessment in which the BBRC operates, and a clarification of purpose is timely.
The aims and objectives of the BBRC.
BBRC aims to maintain an accurate database of records of the occurrence of rare taxa in Britain, in order to enable individuals or organisations to assess the current status of, and any changes in, the patterns of occurrence and distribution of these taxa in Britain
To support this aim, BBRC will strive to:
- Work closely with County Recorders, Bird Observatories and observers to ensure that all records of rare taxa are submitted to this database;
- Provide interested parties with an accurate and complete annual report detailing records of rare taxa in Britain;
- Continue to vet all records of rare taxa in an independent, open, rigorous and consistent manner, and to provide observers with feedback on the assessment process as appropriate;
- Continue to develop and publish criteria for the identification of rare taxa and to provide relevant information to other observers that wish to do this in partnership with the Committee.
As part of the process of reviewing our constitution, we re-examined our aims and thus developed a series of objectives; but even these will need refinement as we test them against current issues. This short article sets out our present philosophy and invites feedback which we can consider in the development of our constitution. Table 1 presents the aims and core objectives of BBRC. In addition, we have a series of other roles including, for example, assisting county committees in the compilation of local avifaunas by undertaking reviews, where requested, of post-1950 records to ensure the accuracy of both the avifauna and our database.
Although our key objectives seem reasonably straightforward, they suggest that there is clear agreement about what constitutes rare birds or ‘rare taxa’. This is not necessarily the case and some working principles are essential. The rest of this article discusses three specific points:
- Why not ‘rare birds’?
- What do we mean by ‘taxa’?
- What do we mean by ‘rare’?
Why not ‘rare birds’?
This question, why we use ‘rare taxa’ rather than ‘rare birds’, is undoubtedly the easiest to answer. Put simply, this is because we are interested in groups of birds, whether species or subspecies, which are vagrants to this country – rather than rare plumage or structural variations in common birds, which have no geographical relevance. For example, xanthochromic Wood Warblers Phylloscopus sibilatrix are a rare plumage variant, where the warbler is almost entirely yellow. This variant is reported in Britain less frequently than, for example, Radde’s Warbler P. schwarzii but, as it occurs sporadically throughout the range of Wood Warbler and has nothing to do with vagrancy, it is not a matter for BBRC.
What do we mean by ‘taxa’?
The lumping and splitting debates, with which we are all currently familiar, reflect a much wider discussion about species concepts, and are relevant not only to birds but throughout the biological world. As we are interested in geographical vagrancy, it seems sensible for us to consider both species and subspecies which occur as vagrants in Britain. Historically, this coverage has not been complete, so that BBRC has considered some rare subspecies but not others. Problems can, however, arise when subspecies (or races – we take these terms to be interchangable) are elevated to species status. For example, BBRC has traditionally assessed records of ‘Siberian Stonechats’ Saxicola torquata maura so that it would be easy to sort out its status should it ever be split from Common Stonechat. This was not true of Hume’s Warbler P. humei; so that when it was separated from Yellow-browed Warbler P. inornatus, our knowledge of the historical status of Hume’s Warbler in Britain was incomplete.
Because the debate on what is or is not a species is largely irrelevant when considering vagrancy we feel that we should consider all diagnosable taxa that meet our statistical criteria of rarity. In this situation, it becomes irrelevant whether the taxon is presently considered to be a species or a subspecies. Although it may seem like dodging the arguments, the debate about whether or not to split is not the concern of BBRC; our role is confined to assessing whether we can be sure that the taxa is the one being claimed. The critical thing to us is not whether it is a species but whether we can identify it – whether it is diagnosable. Even here we face a problem of exactly how we define diagnosable, as it must be clear to all readers that there are a lot of described subspecies that are not universally recognized by relevant authorities. In some cases it is clear that at least one age or sex of a subspecies is consistently diagnosable on present knowledge, while the identification of some other forms is currently evolving or being tested. It is equally clear, however, that some subspecies merely represent a cline(s) across the species’ range, or that the subspecies are very poorly differentiated. . It is the first two cases that BBRC are interested in and we will continue to work with the BOURC and the birdwatching public to establish whether subspecies are consistently diagnosable and if so, to clarify what the relevant identification features are.
What do we mean by ‘rare’?
This is another difficult question to answer. BBRC aims to record the status of rarities in Britain and has guidelines for the statistical threshold for when a taxon might no longer be considered rare. Although we do not always apply these to the letter, because there are a range of complicating factors that need to be considered, these guidelines are the starting point for every decision about what we consider. These are that:
- There should have been at least 150 in the last ten years; and
- There should have been more than ten in at least eight of the last ten years.
Point (b) is necessary to prevent eruptive species such as Arctic Redpoll Carduelis hornemanni being dropped – in most years these are genuinely rare, but occasionally they might comfortably pass the ten-year numerical threshold in a single year! Perhaps the real debate is whether we judge our statistical requirements by prevalence – the number of records in a year– as opposed to incidence – the number of new individuals in a year (i.e. excluding remaining or returning birds)?
Incidence tells us about vagrancy patterns whilst prevalence best reflects birders perceptions of how rare a species is in Britain. With some species and records it is easy to decide whether to use incidence or prevalence [Note 1] but in others the information we use to make important decisions on status and, indeed, whether BBRC continues to consider a species is little more than guesswork [Note 2, Note 3] There are good arguments for the use of both incidence and prevalence, but in deciding between the two we should also remember that returning or wandering individuals allow birders to become more familiar with a taxon Previous experience is a factor in the assessment process as it undoubtedly helps people to find and identify further individuals. In a nutshell BBRC is trying to determine: which forms are diagnosable; the criteria that we consider necessary to identify them; and whether they are rarities.
In the next few years, BBRC will continue to clarify which rare taxa are diagnosable, and what criteria should be used to determine their rarity. As appropriate, we will then request and assess records of such birds. From this we may find that, for example, there are more than ten records per year of ‘Black-bellied Dipper’ Cinclus c. cinclus; if so, then it does not fulfil our criteria as a rarity, and it is most appropriate for it to continue to be handled by county committees. We may well find that ‘white-headed’ Long-tailed Tits Aegithalos caudatus are rarer than Penduline Tits Remiz pendulinus and should thus be assessed by BBRC. Given the fact that BBRC can assess only a finite number of records a year, how will you, the birding public, respond if we adopt a lower threshold of occurrence before a species is dropped from consideration? What about a threshold that could see species such as Radde’s Warbler and Olive-backed Pipit Anthus hodgsoni dropped from the rarities report in favour of rare subspecies?
BBRC will continue to work through these issues in a consistent and pragmatic way, but we would welcome feedback on all of these matters. After all, if it were not for birders submitting records to be assessed, BBRC would not exist and we would not have a complete national archive detailing the status of rarities in Britain. BBRC is a means to an end, not an end in itself. This is why our primary objective is to ‘work closely with County Recorders, Bird Observatories and observers to ensure that all records of rare taxa are submitted…’.
Colin Bradshaw, Paul Harvey & Jimmy Steele, on behalf of BBRC
c/o 9 Tynemouth Place, Tynemouth, Tyne & Wear NE30 4BJ
Note 1
BBRC tends to make an assumption that a first-year bird in one winter is probably the same individual as an adult returning to the same location the following winter. Such assumptions are comparatively safe when considering something like the Laughing Gull Larus atricilla at Newcastle hospital in 1984-86 as there were no other individuals in the area;
Note 2
BBRC has more problems with ‘Black Brants’ Branta bernicla nigricans. We are aware of series of records in areas such as north Norfolk or Hampshire that we believe may relate to returning individuals. There may, however, be up to seven individuals in the Lincolnshire/Norfolk goose flocks. These flocks are very mobile and move between Norfolk and Lincolnshire. Because of this we are never really certain whether the Lincolnshire birds are different to the Norfolk birds and how many there are in any one year. Consequently, trying to make rational decisions about the number of returning individuals and whether last year’s first year birds are now this year’s adults is even less precise. Yet we try to use these very numbers to decide whether we should continue to consider Black Brant as a rarity
Note 3
Birds such as White Stork Ciconia ciconia present special problems. The combination of size, ease of identification, public familiarity and its predisposition for appearing in areas frequented by humans means that a large proportion of occurrences are reported either by birders or the general public. Whilst we can track some wandering individuals, other series of records could reflect multiple arrivals or a single individual. In the case of White Stork escaped individuals further complicate the situation. There has been a tendency to disregard any that is ringed, as an escape, but recent events of a pair ringed in France appearing in Britain show this is not the case and, indeed, a high proportion of the wild population of Holland is ringed. How does BBRC decide whether the number of ‘real’ White Storks has dropped to such a level that we should re-consider the species as a rarity?
