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A comparison of wing length and primary length as size measures for small passerines a


A.G. Gosler , J.J.D. Greenwood , J.K. Baker b

& J.R. King



Department of Zoology, Edward Grey Institute of Field Ornithology, South Parks Road, Oxford, OX1 3PS, UK b

British Trust for Ornithology, National Centre for Ornithology, The Nunnery, Norfolk, IP24 2PU, UK c

Department of Zoology, Edward Grey Institute of Field Ornithology, South Parks Road, Oxford, OX1 3PS, UK Published online: 11 Apr 2011.

To cite this article: A.G. Gosler , J.J.D. Greenwood , J.K. Baker & J.R. King (1995) A comparison of wing length and primary length as size measures for small passerines, Ringing & Migration, 16:2, 65-78, DOI: 10.1080/03078698.1995.9674095 To link to this article: http://dx.doi.org/10.1080/03078698.1995.9674095


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Ringing & Migration (1995) 16, 65-78

A comparison of wing length and primary length as size measures for small passerines

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A report to the British Ringing Committee A.G. GOSLER Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford, OX1 3PS, UK J.J.D. GREENWOOD British Trust for Ornithology, National Centre for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU, UK J.K. BAKER British Trust for Ornithology, National Centre for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU, UK J.R. KING Edward Grey Institute of Field Ornithology, Department of Zoology, South Parks Road, Oxford, OX1 3PS, UK The length of primary 3 (ascendent) has been strongly promoted in some countries as a better measure of body size than wing length for small passerines. This is because it is believed to offer a more repeatable measure and to be safer for the bird. New data from the British ringing scheme suggest that, for trained observers, repeatability of primary 3 length is no better than thatfor maximum chord wing length. Furthermore, sexual dimorphism in primary 3 length is perhaps less marked than that in wing length, so that it may be a poorer metric to use to discriminate sexes. We can find no evidence supporting the use of primary 3 length in preference to wing length on grounds of bird welfare.

~\ A Whatever the use to be made of them, it is V V important that measurements should be repeatable. Any variation between successive measurements by the same observer will reduce the precision with which, for example, mean values of a variable for a population of birds can be calculated. Any differences between observers make it impossible satisfactorily to compare their observations. Consider, for example, measurements taken from two different wintering populations, which provide different means and therefore suggest that the birds originate from different wintering areas: that conclusion is only valid if those studying the birds in the two populations measure them without bias relative to each other. Repeatability is not, however, the sole criterion. It is also important that the measurements should be effective for the use to which they are to be put. Thus, although counting the number of toes on a bird is much more repeatable than measuring its wing length, toe-counts are ineffective at distinguishing the sexes whereas wing length may differ sufficiently between the sexes to be a useful discriminator • even when the measurement of wing length differs slightly between observers. In this paper we consider

alternative wing size metrics, in terms both of repeatability and of power to discriminate the sexes. For many years, wing length, measured on the closed wing from the carpal to the tip of the longest primary, has been taken as a standard measure of body size in birds, especially small passerines (Spencer 1984). This metric has, however, been criticised as having poor repeatability (Spencer 1965, Berthold & Friedrich 1979, Jenni & Winkler 1989, Svensson 1992). The problem stems from the fact that the wing is not only curved in two planes but is also flexible. Furthermore, three different measures of wing length have been described: (a) unflattened wing (minimum chord), (b) flattened but not straightened wing, (c) flattened and straightened wing (maximum chord) (Spencer 1984). It is not sensible to include differences between the methods when assessing the repeatability of wing length because that problem can easily be overcome by standardising on one method. Using method (c), it is possible to achieve absolute agreement between 98% of successive measurements of small passerines made by the same observer (Gosler 1987) and recent workers in Europe

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have mostly used this method because it appears more repeatable than (b), which is itself more repeatable than (a) (Spencer 1965, Svensson 1992). In North America, however, many workers still use (a) because it produces results more comparable with those from skins (Pyle etal. 1987). Jenni & Winkler (1989) list various references that document the imperfect repeatability of wing length, especially in terms of between-observer differences. Indeed, all ringers are aware of such differences, particularly the general tendency of inexperienced trainees to measure l-2mm short compared with experienced ringers. The key question is not, however, how repeatable (or otherwise) is wing length but whether there is a better alternative. Berthold & Friedrich (1979) have addressed this question by comparing the maximum chord method with a measure of the length of the third primary (counting ascendently). The latter was obtained by inserting a pin mounted on a rule between the second and third primaries near their bases, sliding it to the feather base, flattening the third primary and measuring it on the rule (see illustrations in Berthold & Friedrich 1979, Jenni & Winkler 1989, Svensson 1992). They asked nine experienced and seven inexperienced observers each to measure the same 23 Tree Sparrows Passer montanus. The differences between the means of the measurements made by different observers were substantially greater for wing length than feather length, as was the average difference between the experienced and inexperienced observers. A simple interpretation of this is that the repeatability of wing length is less than that of primary 3 length and that wing length is a metric that is particularly affected by experience or training. The results presented by Berthold & Friedrich show something more, however, in that the differences between the observers' means (measured as variances, in mm2) are:

Wing length Primary 3 length

Inexperienced Experienced 0.07 4.59 0.07 0.02

While the other three values are consistent within the limits of sampling error, that for wing length measurements made by experienced observers is hugely different (note 1). We suggest that all of the observers were consistent when measuring primary 3 length because they were all shown how to do it at the start of Berthold & Friedrich's investigation; that the inexperienced observers were consistent when measuring wing length for the same reason; and that the experienced observers showed such big differences in mean measurements of wing length because they had individual habits in taking this measurement that had been established before they took part in Berthold & Friedrich's study. Our interpretation of these findings suggests that the repeatability of wing length measurements may be much improved by careful training. Standards of ringing procedures have been regulated in Britain and Ireland for more than 15 years by a rigorous training system, during which ringers are examined by independent sponsors and are encouraged to attend training courses (Ringing Committee 1987). During this training, strong emphasis is placed on obtaining standard wing length (maximum chord) measurements, following Spencer (1984). We might thus expect British and Irish ringers to make more repeatable wing length measurements than those in countries with less rigorous training procedures. No formal tests of repeatability of wing length measurements (nor its comparability with the repeatability of primary 3 length measurements) have, however, been carried out in Britain. Testing overall repeatability is not easy, as ringers within the same group will have similar habits. We report here on trials that were carried out during 1989-91 at several British Bird Observatories, to which visitors came from many different training backgrounds. The trials were designed to compare repeatability of wing length (= maximum chord). In addition, comparison with feather length measurements was made on two ringing courses, also involving ringers from various backgrounds. Finally, we also report on the relative power of wing length and primary 3 length to discriminate sexes in a number of

Size measures for small passerines species, concerning which there have been no previous investigations but which we believe to be fundamentally more important than repeatability itself.

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METHODS Observatory trials: assessment of wing length repeatability Between 1989 and 1991, British Bird Observatories were asked by the Biometrics Working Group of the BTO Ringing Committee to submit sets of repeated measurements of passerines processed during routine operations. In addition to wing length, data were requested for a range of size and condition measures, some of which were previously unknown to the observers and for which detailed standard instructions were issued. For wing length, observers were asked to measure maximum chord as they had been trained. We report here only on wing length, which was to be taken as trained, to the nearest mm, although reference was made to Spencer (1984) in the instructions. Results for repeatability of other biometrics will be presented elsewhere. Observatories were also issued with sets of standard recording sheets on which both a visiting ringer's measurements and those of an appointed 'standard' observer (usually the observatory warden) could be recorded. Visitors always processed each bird first, followed by the standard observer who measured independently. While some other protocol might have ensured greater independence of observation, this would have required birds to have been kept for longer periods than was acceptable in consideration of their welfare. Nevertheless, all standard observers were experienced instructors and even if they had access to visitor data they were unlikely to be influenced by such knowledge. This view is strongly supported by some rather low repeatabilities obtained for other biometrics not reported here. Wing length and primary 3 length repeatability trials Trials were carried out to assess the repeatability of wing length and primary 3 length during two weeklong bird ringing courses held at Spurn Bird Observatory, Humberside during October 1992 and 1993. As trials were carried out in the same way in the two years, the data have been pooled. In all, 20 ringers (termed visitors, as above), varying in experience from recent trainee to experienced sponsor, made relevant measurements on these two courses, though one of them contributed data for only one bird, so has been omitted from most of the analyses (for which multiple observations were needed from each observer). All of these observers had received some training in the measurement of maximum chord prior to the course but from more than ten trainers distributed widely across the UK. Only three had previously measured primary length. During 1992 a range of species was sampled, while in 1993 measurement focused on just four species,


two small (Robin and Dunnock) and two large (Song Thrush and Redwing). Each bird was measured twice by the visitor, once to determine wing length and once to determine primary 3 length. The latter was taken using the standard rule described by Jenni & Winkler (1989) and supplied by Vogelwarte Sempach, Switzerland. All measurements were taken to the nearest 0.5mm. After processing, each bird was re-measured by a single 'standard' observer (AGG) without reference to the visitor's measurement. Errors greater than 8mm were found, invariably, to be due to a misreading of the rule by the visitor. Since this was not the source of error under investigation, these measurements were rejected. They comprised less than 2% of the measurements. Most occurred in respect of primary 3 rather than wing length. The data were examined in relation both to the experience of the visitor (indicated by their licence status, T • trainee, C provisional and A - full, see Spencer 1984) and to the day of the course (to assess whether visitors improved during the week). Sexual dimorphism of wing length and primary 3 length To assess the value of wing length for discriminating sexes, we present data from 15 passerine species from 12 genera in seven families). Jenni & Winkler 1989) noted that across species, the lengths of different primaries correlated with wing length to different degrees. In some, primary 3 was best, in others p4 or (rarely) p2, depending on which formed the wing point. Our data, therefore, include some species in which p3 was best correlated with wing length and some in which p4 was best (from Jenni & Winkler 1989). Two data sets were amassed independently by AGG (11 species in the UK) and JRK (eight species in the UK and Mallorca, Spain). For the analysis, we have pooled the data for both observers and for all populations within a subspecies. In most cases, sex was determined by plumage. Species in which this was not possible were sexed by the presence of a brood patch (female) or cloacal protuberance (male) during the breeding season (Svensson 1992). Age classes were pooled since sample sizes were small and since the effect of pooling should be the same for the two metrics. Statistical methods The statistical methods used are outlined briefly as the results are presented below, with details relegated to the appendix. Note that we have not used the standard measures of "repeatability" (Harper 1994) or "reproducibility" (Barrett et a!. 1989), finding more direct measures of the differences between observers to be more useful for our current purposes.

RESULTS Observatory trials: assessment of winglength repeatability We obtained a total of 1,215 pairs of measurements from the 20 species for which


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