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Wing Length Changes ofthe Knot with Age andTime since MoultMichael W. Pienkowski & C.D.T. MintonPublished online: 23 Jun 2009.

To cite this article: Michael W. Pienkowski & C.D.T. Minton (1973) WingLength Changes of the Knot with Age and Time since Moult, Bird Study, 20:1,63-68, DOI: 10.1080/00063657309476359

To link to this article: http://dx.doi.org/10.1080/00063657309476359

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Wing Length Changes of theKnot with Age and Time sinceMoult

by Michael W. Pienkowski and C. D. T. Minton

Generations of ornithologists have used the wing length as astandard linear measurement for the discrimination of sexgeographical variation, etc.—but how reliable a parameter is it?This paper considers some of the problems brought to light byremeasuring a sample of Knots at different ages and at differenttimes after the annual moult.

WING LENGTH HAS FREQUENTLY been used as an aid in distinguishingbetween populations of birds. However, before this method can be con-sidered reliable two basic requirements must be fulfilled :

(1) that the technique used for measurement gives consistent results (a) for anindividual measurer, and (b) between all measurers, and

(2) that the wing length distribution of a population either remains constantor changes in a predictable manner (a) during one inter-moult period, and(b) from year to year.

This paper uses data on the Knot Calidris canutus, collected since 1967by the Wash Wader Ringing Group, to illustrate the above considerations.

TECHNIQUESEarlier problems concerning inconsistencies of technique have been mini-mised by measuring the maximum wing length (Evans 1964), involvingthe flattening and straightening of the primaries (method iii in Spencer1965). This method was used for all measurements described below and isvery consistent for an individual measurer. During the measuring of acatch of 177 Knots on the Wash on 31 January 1971, 40 birds taken atrandom were given to the measurer a second time, and he was unaware ofwhich birds he was remeasuring. The change from first to second lengthdid not exceed 1 mm. for 31 of the birds. The mean change was not signifi-cantly different from zero (student's t-test, P>0.1) and there was no sig-nificant difference between the numbers increasing and those decreasing(chi-squared test, P>0.1). Most of the variance is due to errors in record-ing when large numbers of birds are being handled rapidly, rather thanto the measuring technique itself. On 4 December 1971 the same meas-urer was checked under more ideal recording conditions and, of 73 birdsmeasured twice, 70 (96%) had differences not exceeding 1 mm.

In order to test for any difference between wing length measured by twoindividuals, each measured half of a catch of 352 Knots on 15 November1970. There was no significant difference between the results (student'st-test, P>0.1). It can therefore be concluded that, on reasonably largesamples, the technique described above can give wing length values con-

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WING LENGTH CHANGES OF THE KNOT

(WW ) 11 16 u a l

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sistent for an individual measurer and between checked measurers. All themeasurements used in the following analysis were taken by one or otherof the two individuals compared above.

RESULTSVariation in wing length during an inter-moult periodWing lengths of first-year birds tend to be shorter than those of adults(Stewart 1963). Metropolsky (1961) suggested the use of wing length as ameans of distinguishing between first-year, second-year and adult Oyster-catchers Haematopus ostralegus. However, it is usually assumed that,once adult, a bird has a constant wing length (possibly excepting a periodimmediately prior to moult, when feathers may be heavily abraded). Nis-bet (1967) noted a decrease in mean wing length of 3.6 mm. (5%) overnine months in Pallas's Grasshopper Warbler Locustella certhiola, whilePrater (1970) demonstrated a decrease of 4 mm. (3%) in BlackbirdsTurd us merula over a similar time period. However, both estimates werebased on means for the samples caught, there being no confirmation frommeasurements of retrapped birds.

The wing lengths of 65 adult Knots caught and measured twice duringone inter-moult period are shown in Figure 1 where each line representsone bird and joins the two values for wing length on the two dates: 51 ofthe 65 (78%) decreased in wing length and 10 increased (four remainingconstant). This downward trend was highly significant (chi-squared test,P<0.001). Most of the lines are fairly parallel despite the fact that theperiods spanned occur in all parts of the year, suggesting that the rate ofdecrease in length is fairly constant throughout the inter-moult period.

An estimate of the mean change in wing length per month has beenobtained by regression of the change in wing length between first andsecond measurings on the time interval, applying the constraint that theline should pass through the origin (see Williams 1959). This gives a valueof 0.68 mm. decrease per month which is significantly different from zero(student's t-test, P<0.001).

Data are available for only 19 juvenile Knots but it appears that a simi-lar shortening occurs. There is a mean decrease of 0.45 mm. per monthbut, because of the small sample, this figure is not as reliable as that forthe adults. Although the decrease was not significantly different fromzero (student's t-test, P>0.05), there was a significant tendency to decrease(chi-squared test, P<0.001) and there is no apparent reason that thechange should differ greatly from that of the adults. Similar changes havebeen calculated for Sanderling Calidris alba (Williams and Minton, inprep.), 20 adults decreasing in wing length by a mean of 0.30 mm. permonth and 10 juveniles by 0.64 mm. per month. Both changes are signifi-cantly different from zero (student's t-test, adults P<0.01, juvenilesP<0.001) and from one another (P<0.001).

Adult Knots complete the growth of new primaries at the end ofOctober or beginning of November and moult their longest primary inlate September (Wash Wader Ringing Group data). In the intervening 101months the shortening is 7-8 mm. (4% of wing length). Corresponding

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18-

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WING LENGTH CHANGES OF THE KNOT

Excess wing length ( mm )

Figure 2. Excesses of observed over predicted wing lengths at the second measuringoccasion for 65 adult Knots recaptured after one moult.

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22 O1-1

28 26 24• ,i,11 171 ,

4 6 8 10 12 14 16 18

Excess wing length (mm)

Figure 3. Excess of observed over predicted wing lengths at the second measuringoccasion for 85 adult Knots recaptured after two moults.

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figures for Sanderlings are 2.5% for adults and 5% for juveniles. It is notclear whether this shortening, which is fairly constant throughout theyear, is due entirely to abrasion. Abrasion, although acting during thewhole year, becomes most noticeable late in the life of the feathers which,in waders, commonly become very worn in early autumn. Similar wearingof feathers has been reported in Guillemots Uria aalge and gulls Larusspp. (Kennedy and Melville 1971, Bourne 1971), and Grant (1971) sug-gested that the tips of gulls' wings frequently become extensively worn.

An additional cause could be a change in the chemical or physicalnature of keratin, the structural protein of feathers. There is some slightevidence for denaturing of feather keratin in that several Knots caught inNovember 1970 and February 1971 had quite obviously failed to moultand the primaries were deteriorating considerably with large parts of thefeathers disintegrating. Experiments are being undertaken to investigatethe extent of contributions made by these possible causes of shortening.

Year to year changes in wing lengthWith the possibility of 'correcting' a wing length distribution to that of

the same birds at a different time of year, it is now feasible to investigateyear to year changes for the birds : 65 adult Knots have been measuredtwice with one intervening moult and 85 with two intervening moults.For all of these birds the 'expected' wing length for the month of thesecond capture has been calculated from the first wing length and the pre-dicted change of 0.68 mm. per month. The differences of the 'expected'values from those observed are shown for the two groups in Figures 2 and3. This prediction is only of use if the variation in the predictor is lessthan the variation in the wing length differences of pairs of birds taken atrandom out of the population. The F-distribution has been used to com-pare variances for the differences between observed and expected valuesin Figures 2 and 3 with an estimate of the variance for the differences inwing length between pairs of birds taken at random (twice the variance forwing length in a large sample catch). In each case the variance was signifi-cantly less than that of the random situation (P<0.001 for Figure 2;P<0.01 for Figure 3).

It is at once apparent from Figures 2 and 3 that, in most cases, observedlength is greater than expected and therefore the wing lengths of the birdsare increasing with age. After one moult the mean increase is 1.7 mm. andafter two moults 3.8 mm. (1.0% and 2.2% respectively). These differencesare significantly different both from zero and from each other (student'st-test, P<0.001 in each case). A similar procedure used on 14 birds givesa value of 6-7 mm. (4%) increase from new juvenile feathers to newsecond-year feathers. This increase is significantly different from zero(student's t-test, P<0.02). Thus the wing length immediately after com-pletion of moult appears to increase from juvenile to second-year plumageas previously described by other authors (Stewart 1963, Metropolsky1961) but also about 1% additionally in future years. This will lead toadditional components in a wing length distribution, tending to blur differ-ences between populations, if the differences are not large.

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It should be noted that two factors affect the differences in wing lengthbetween first-year, second-year and adult birds. The first is the differencein new feather wing length which, as described above, increases with age.In addition, throughout much of the year the primaries of an adult may be1-2 months newer than those of a second year and 3-4 months newer thanthose of juveniles which fledge in late July (Salmonsen 1950), as the longestfeather becomes fully grown in adults in early November and in second-year birds in mid-September (Wash Wader Ringing Group data). Thesetwo factors are additive and, if scaled up to the size of an Oystercatcher,would account for the differences between adult and second-year birds ob-served by Metropolsky (1961).

The wing length of a bird is affected by numerous factors and, becauseof this, is not an ideal parameter for separating populations. However, itdoes appear to be the major metric method for doing so in many casesand, as the main changes in length appear to be regular, it should bepossible to correct for them in cases where a large amount of data areavailable for the populations.

ACKNOWLEDGEMENTSWe would like to thank the members of the Wash Wader Ringing Group whocollected the data used in this paper. We are also grateful to Drs D. Aikman, C. F.Mason, and P. I. Stanley and Messrs H. Boyd, A. J. Prater and D. I. Sales for help-ful comments on an earlier draft.

SUMMARYAdult Knots' wing lengths decrease by a mean of 0.68 mm. per month (7-8 mm. peryear, which is 4%). Similar decreases have been calculated for juvenile Knots andfor adult and juvenile Sanderlings (2.5% and 5% respectively for the last two).This decrease appears to be linear but it is not clear whether it is due entirely toabrasion.

As first-year and second-year birds are likely to complete primary feather growthearlier than adults, their wing lengths during most of the year will have decreasedproportionally more. New feather wing length of juvenile Knots increases 6-7 mm.(4%) after their first moult and there appears to be an increase of about 1.8 mm.(1%) at later moults.

REFERENCESBOURNE, W. R. P. 1971. Feather wear in Guillemots. Brit. Birds, 64:236.EVANS, P. R. 1964. Wader measurements and wader migration. Bird Study, 11:23-38.GRANT, P. J. 1971. The wing-tip patterns of gulls. Brit. Birds, 64:285-287.KENNEDY, R. 5. AND D. MELVILLE. 1971. Further examples of apparently abnormal feather wear.

Brit. Birds, 64:229-230.METROPOLSKY, 0. B. 1961. Determining the age of birds by the length of the remiges. Ornithologiya,

4:416-417. (In Russian.)NISBET, I. C. T. 1967. Migration and moult in Pallas's Grasshopper Warbler. Bird Study, 14:96-103.PRATER, A. J. 1970. The variation of the weights and wing lengths of Blackbirds at Rye Meads. 5th

Report of the Rye Meads Ringing Group, 16-24.SALMONSEN, F. 1950. The Birds of Greenland, 2:222-229. Kobenhavn.SPENCER, R. 1965. The Ringer's Manual. British Trust for Ornithology, Tring.STEWART, T. F. 1963. Variation of wing length with age. Bird Study, 10:1-9.WILLIAMS, E. J. 1959. Regression Analysis. New York.

M. W. Pienkowski, School of Biological Sciences, University of EastAnglia, Norwich, Norfolk, NOR 88C.

Dr C. D. T. Minton, 65 St. John's Hill, Shenstone, Lichfield, Staffordshire.

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