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Andre Hermansson
Degree project for Bachelor of Science in
Biology
Zoology 15 hec Spring 2013
Department of Biological and Environmental Sciences University of Gothenburg
Examiner: Johan Höjesjö
Department of Biological and Environmental Sciences
University of Gothenburg
Supervisor: Staffan Andersson
Department of Biological and Environmental Sciences
University of Gothenburg
Growth stress and costs of tail ornamentation in Buff-shouldered widowbird
(Euplectes psammocromius)
Sexual dimorphism and costs of tail
ornamentation in Buffshouldered widowbird
2
Abstract
Costs of male tail ornamentation were investigated in the little studied buff-
shouldered widowbird (Euplectes psammocromius), a species found only in the
grass covered highlands of SW Tanzania and the Nyika Plateau of NE Zambia
and N Malawi. Growth bars (indicating daily growth) and fault bars (indicating
growth stress) were counted in tail feathers collected from males in nuptial
plumage from two breeding seasons, in order to get a measure of feather growth
and fault rate, fault rate being a measure of stress. Growth rate and fault rate were
correlated to each other, tail length and a standardized measure of body condition.
Elongated tail feathers appear to be costly to produce, since a positive relationship
was found between tail length and fault rate. Furthermore, increased growth rate
seems to contribute even more to the cost of producing tail feathers than increased
tail length, particularly increased growth rate in the early phase of feather growth.
The fault bar score (a measure of the absolute amount of fault bars) was greater in
the late (last formed) half of the feather. Final tail length was weakly but
significantly positively related to body condition, suggesting that tail length may
serve as a male quality advertisement in the context of female mate choice.
Sammanfattning
Kostnaden av stjärtfjäderornament undersöktes hos den föga studerade buff-
shouldered widowbird (Euplectes psammocromius), en art som endast återfinns i
de gräsbeklädda höglandsområdena i SV Tanzania och Nyika Platån i NÖ Zambia
och N Malawi. Så kallade ’growth bars’ (som indikerar dygnstillväxt) och ’fault
bars’ (som indikerar tillväxtstress) räknades i stjärtfjädrar insamlade från hanar i
praktdräkt under två häckningssäsonger för att därigenom få ett mått på
tillväxthastighet och ’fault rate’, det senare som ett mått på stress. Fjädermåtten
korrelerades sinsemellan, samt med stjärtlängd och ett standardiserat
konditionsmått. Stjärtfjäderornamentet verkar vara kostsamt att producera då ett
positivt samband hittades mellan fjäderlängd och faultfrekvens. Vidare verkar
ökad tillväxthastighet bidra ännu mer till kostnaden för fjäderproduktionen än
ökad stjärtlängd, framförallt ökad tillväxthastighet tidigt i fjäderbildningen. ’Fault
bar score’ (mått på den absoluta mängden av ’fault bars’) var högre i den inre (sist
bildade) halvan av fjädern. Kondition var svagt men signifikant positivt korrelerat
med stjärtlängd vilket antyder att stjärtlängd kan fungera som en indikator av
hanlig kvalitet i samband med honligt partnerval.
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Introduction
Study species
The widowbirds and bishops
together form the genus
Euplectes, a group of 17 small
seed-eating Afrotropical
weaverbirds (Staffan
Andersson 1994). The buff-
shouldered widowbird,
Euplectes psammocromius, is
a very little studied (Staffan
Andersson, personal
communication, Mars, 2013)
species of widowbird, that is
only found on the Nyika
Plateau of NE Zambia and N
Malawi and the grass covered
highlands of SW Tanzania,
northeast to Njombe and
Iringa (Fig. 3), where it’s
locally common (Fry 2004).
Like all widowbirds (Staffan Andersson 1994) it has a strongly
sexually dimorphic breeding plumage, breeding males having
black body plumage, black elongated tail feathers and bright
yellow buff shoulder patches, while non-breeding males and
females have a duller, brownish plumage (Fry 2004). (See
Figure 2 for an illustration of the different plumages).
Little is known about its’ breeding habits, but it probably is
polygynous and territorial, with breeding males displaying
their elongated (ca 30 cm) tails making them look like several
tails, in a slow display flight (Fry 2004).
All sexually selected characters are expected to continue
evolving until the mating advantages are balanced by an
opposing cost (Fisher 1930). Unfortunately, these balancing
costs and constraints have been much less studied than female
mate choice based on secondary sexual characteristics
(Andersson 1994). Knowledge of these costs is crucial in
understanding why, for instance, the tail feathers of male
widowbirds are not even longer than observed, given the
assumption that females prefer longer tails. Such a female bias
for longer tails has been found in several other widowbird
species; the long-tailed widowbird, E. progne (Andersson 1982), Jacksons widowbird, E. jacksoni
(Andersson 1992), red-collared widowbird, E. ardens (Sarah R. Pryke 2001) and red-shouldered
widowbird, E. axillaris (Sarah R. Pryke 2002).
Figure 2. Breeding male, non-breeding male
and female, E. psammocromius (Fry 2004)
Figure 1. The distribution of E. psammocromius (BirdLife 2013)
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Study objectives
This study aims to discover if there are physiological costs of growing elongated tail feathers in breeding
(nuptial) males, and if the final tail length is dependent on male condition, which would suggest that it
could be a male quality advertisement in the context of female mate choice. My hypothesis is that there
likely are significant costs to growing long tail feathers and that it probably is condition dependent, as is
the case in the closely related species, Jackson’s widowbird, E. jacksoni (Andersson 1994). To test this,
various feather measurements on collected tail feathers were performed, and these measures where then
combined with several biometric (body part) measurements of the birds.
Methods
Sampling sites
Feather samples and biometrical data were collected during the course of two breeding seasons,
December-February 2010/2011 and December-February 2011/2012. The study area is located in the
highlands of south-western Tanzania; samples were taken from two study sites, Mtitu (08°12’S 35° 48’E)
and Mtanga (08°08’S 35°50’E). Both sites are located in the grass covered hills and valleys of Kilolo
district in the Iringa region.
Measurements
The birds were captured at territories
or roosting sites using mist nets or
bow nets and were subsequently
ringed with metal rings. The
biometrical measures used were;
tarsus length, tail length and body
mass. Tarsus length (between the
extreme bending points at toes and
heel) was measured with callipers to
the nearest 0.1 mm. Due to the tarsus
measurements in the 2009/2010 and
2010/2011 studies being made in a
different manner, the values obtained
were slightly shorter than in the
latest 2011/2012 study. Therefore, a
slight correction had to be made
when using the data to compensate
for this methodological difference
(see the ‘Statistics and methods of
data analysis’ part at the end of the Methods section). Tail length was measured to the nearest 0.15 cm
using a ruler, and the number of blood quills was counted. The birds' body mass was weighed to the
nearest 0.1 g, using a Pesola spring balance.
For the feather samples the R4 tail feather (retrix), i.e. the 4th from the middle feather, was selected
from each individual bird. Out of the collected tail feathers, the most intact and readily measurable were
selected for analysis; 62 from the latest 2011/2012 season and 25 from the 2010/2011 season. All
Figure 1. Iringa region, Tanzania (GoogleMapMaker 2013)
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measured feathers were from males with nuptial plumage (black body feathering and elongated black
retrices) except for one eclipse (dull, sparrowy feathering similar to females) male. The attributes
measured in the feathers were; feather length, growth bars and fault bars. Growth bars are alternating light
and dark bands roughly perpendicular to the feather axis (rachis) that denote 24 h cycles, with one dark
band appearing during the day and one light band during the night (Grubb 1989), thus they can be used as
a measure of how long the feather has been growing. Fault bars are translucent bands roughly
perpendicular to the rachis caused by malformation of the barbules, due to malnutrition or other forms of
stress (Riddle 1908, James R. King 1984).
Both the growth bar and fault bar measurements began at a standardized distance (4 cm) from the base
of the feather since, the bars closest to the base were often very difficult to see (due to damaged barbs)
and the feather barbs only start at a certain distance up from the base. The feathers' whole length from tip
to base was measured using a tape measure and the whole length was subsequently divided into two
halves. Growth bars were counted on a light table, first for the whole feather, and then for the early half
of the feather, i.e. the half closest to the tip. Thereafter, the feather was divided into 8 equally long parts
(octiles), which were marked on the rachis. Fault bars were divided into three classes based on their
length (see Appendix 1 for precise definitions) and for each feather octile the number of fault bars of each
class was counted. Both a non-weighted score (i.e. simply the sum of the number of fault bars over all the
octiles) and a weighted score was calculated. The weighted score was calculated by multiplying the
number of fault bars of each class with a class specific weighted factor; 1 for class 1, 2 for class 2 and 3
for class 3, and then summing it all together. Fault bar scores were also calculated for each of the two
halves of the feather by summing the fault bar score of octiles 1-4 for the early half and octiles 5-7 for the
late half (octile 8 is excluded since the measurements started 4 cm from the base of the feather). An
average feather growth rate (mm/day), both for the whole feather and for the two halves separately, was
calculated by dividing the feather length by the number of growth bars (the whole feather length divided
by the total number of growth bars, and half the feather length divided by the number of growth bars of
each half, respectively). A body condition index was calculated as the standardized residuals from a linear
regression of ln(body mass) by 3 x ln(tarsus length)(Sarah R Pryke 2003).
Statistics and methods of data analysis
As previously stated the tarsus lengths differed slightly between the earlier season and the latest one, due
to methodological differences. To account for this, an average length was calculated for the earlier season
and another average for the latest season, subsequently the difference in average between the seasons was
added to all the tarsus lengths from the older season. This enabled data from the different seasons to be
pooled together and be treated as a single data set.
Two different ways of obtaining a standardized fault bar rate were used; one way was by dividing the
fault bar score by the feather length, the other by dividing the fault bar score by the number of growth
bars. This was done to get rid of the dependence of the fault bar score on feather length and feather
growth rate respectively (see Results for the correlations between feather length and fault bar score, and
feather growth rate and fault bar score). A standardized weighted fault bar score for the 1st and 7
th octiles
was obtained by dividing the weighted fault bar score of each octile by the average feather growth rate of
the early and late feather halves respectively. The 1st and 7th octiles were chosen because they are the
furthest apart, thus they should be representative of the early and late growth respectively. Growth rates
had to be used in this case instead of growth bar count, because the growth bars had not been counted for
each octile. This standardization was necessary when comparing the 1st and 7
th octiles (Fig. 9, Results),
because a significantly higher growth rate was found in the late feather half compared to the early (Fig. 8,
Results).
Parametric (Pearson’s r) and non-parametric (Spearman’s ρ, when data did not fulfil normality criteria)
correlation analysis were used to see if any relationships between the ptilometric data (feather
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Figure 2. The realtionship between the weighted and non-weighted bar scores. Linear
regression: R2=0.92, n=85, p < 0.0001
measurements) and the biometrics could be found. To plot significant correlations, bivariate scatter plots
with linear regression lines were used. In the comparisons done between feather halves and between
octiles, Wilcoxon signed-rank test was used.
Results Notes on the selection of data
Because most of the trends using only the data from the 25 individuals from the earlier season are non-
significant when tested, and the trends are virtually the same but significant when using the data from the
62 individuals from the latest season, the decision was made to pool the data from the two seasons and do
the analysis on the combined data set. Feathers with blood quills are excluded because they were still
growing (Andersson 1994), hence their final length would be impossible to know. Two outliers with
extremely short tails, one of them being the eclipse male mentioned before, are excluded from the
analysis, because they were not deemed representative of the nuptial male population of interest to this
study and will only obscure the relationships of interest if included. For the fault bars, only the weighted
values are used in the analysis. This is because the weighted values do not only show a count of fault
bars; they actually say something about the relative size of the different fault bar types. Furthermore,
when the resulting trends using the weighted values were compared to the ones using the non-weighted,
they were always in the same direction but somewhat stronger. This is to be expected if the weighted and
non-weighted scores are tightly correlated which they are (Fig. 4). The two different standardized fault
bar rates yielded very similar trends when used; therefore, only one of them, the one obtained by dividing
the fault bar count by the number of growth bars, is used in the statistical analysis.
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Variation in and distribution of data
Before showing the main results it would be useful to consider the variation in and distribution of some
of the measures most important to this study. The variation among males in tail length, average feather
growth rate and weighted fault bar rate is quite large as is shown in Figure 5. For more detailed numbers
(mean, SD, SE, n) of the aforementioned and other measures see Table 1. Both tail length and average
feather growth rate were normally distributed (Shapiro-Wilk test, n=75, p=0.42, and n=85, p=0.56
respectively), the weighted fault bar rate however was not (Shapiro-Wilk test, n=85, p < 0.001).
Table 1. Variation in some of the ptilometric and biomtetric measures used.
Trait Measure or unit Mean SD SE n
Ptilometrics:
Feather length mm 263 28 3 87
Total number of growth bars Growth bar count for whole
feather
69 9 1 87
Average feather growth rate
(whole feather)
mm/day 3.85 0.33 0.04 87
Average fault bar rate
(weighted)
Fault bar weighted score/total
number of growth bars
0.5 0.3 0.0 87
Biometrics:
Tail length mm 276 29 3 78
Tarsus length (corrected for
method difference)
mm 31.9 1.6 0.2 81
Body mass g 39.9 3.6 0.4 79
Body condition Residual mass index -0.0023 0.0469 0.0056 69
Figure 3. Histograms with error bars and accompanying boxplots with outliers shown as dots, showing from left to right;
the distribution of tail length (mm), average feather growth rate (mm/day) and weighted fault bar rate (fault bar weighted
score/total number of growth bars).
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Correlations with fault bar scores There is a positive relationship between feather length and fault bar score (absolute amount of fault bars),
and feather growth rate and fault bar score (Fig. 6 and 7).
Comparison of growth rate between the two feather halves
There is a significant difference between the two halves (Fig. 8); the late half having a significantly
higher growth rate.
Figure 6. The relationship between weighted fault bar score
(fault bar count multiplied with weight factors) and feather
length (mm). Linear regression: R2=0.16, n=85, p=0.0002
Figure 7. Weighted fault bar score (fault bar count multiplied
with weight factors) plotted against average growth rate
(mm/day). Linear regression: R2=0.15, n=85, p=0.0003
Figure 8. Boxplots comparing the average feather growth rate (mm/day) of the
two feather halves. Wilcoxon signed-rank test, n=85, p < 0.0001
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Comparison of standardized fault bar score between the 1st and 7th octiles
The standardized (see Methods) weighted fault bar score is significantly higher in the 7th than in the 1st
octile (Fig. 9).
Relationships among ptilometrics, and between ptilometrics and biometrics
A significant, though weak, positive
correlation between male tail length
and body condition was found (Fig.
10). No significant correlation can be
found between body condition and
weighted fault bar rate (Spearman's
ρ=-0.0775, n=72, p=0.52). However, a
significant positive correlation exists
between the average feather growth
rate of the whole feather and weighted
fault bar rate of the whole feather (Fig.
11). When the average feather growth
rate of the early half is used,
significant positive correlations
emerge for the following; average
feather growth rate of the early half
against the weighted fault bar rate of
the whole feather (Spearman's ρ=0.41,
n=85, p < 0.0001), average feather
growth rate of the early half against
the weighted fault bar rate of the early
Figure 9. Boxplots comparing the standardized weighted fault bar scores
of the 1st and 7
th octiles. Wilcoxon signed-rank test, n=85, p < 0.0001
Figure 10. The relationship between body condition (residual mass index,
see Methods) and tail length (mm). Linear regression: R2=0.09, n=68,
p=0.0108
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Figure 11. Weighted fault bar rate of the whole feather plotted against average
feather growth rate (mm/day) of the whole feather. Linear regression: R2=0.23,
n=85, p < 0.0001
half (Spearman's ρ=0.28, n=85,
p=0.0102), and the average
feather growth rate of the early
half against the weighted fault
bar rate of the late half
(Spearman's ρ=0.34, n=85,
p=0.0016).
When the average feather
growth rate of the late half is
correlated against the same
measures (i.e. weighted fault bar
rate of the whole feather,
weighted fault bar rate of the
early half, weighted fault bar rate
of the late half); there is a
significant positive result for the
correlations with the fault bar
rate of the whole feather
(Spearman's ρ=0.32, n=85,
p=0.0026) and the fault bar rate
of the late half (Spearman's ρ=0.40, n=85, p=0.0001), and a non-significant positive correlation with the
fault bar rate of the early half (Spearman's ρ=0.19, n=85, p=0.0875). In addition to growth rate, tail length
also positively correlates with fault bar rate (Fig. 12).
Figure 12. The weighted fault bar rate (weighted fault bar score/number of growth bars)
plotted against tail length (mm). Linear regression: R2=0.10, n=75, p=0.0054
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Discussion Is tail ornamentation stressful to produce?
Since there is a positive relationship between feather growth rate and fault bar rate (Fig. 11, Fig. 12), it
appears to be stressful to grow feathers more quickly. Furthermore, the growth rate in the early phase of
feather growth appears to influence the fault bar rate the more, since the correlation coefficient is stronger
for the correlation of the fault bar rate (of the whole feather) with the early half than with the late half
(Spearman's ρ=0.41 and Spearman's ρ=0.32 respectively). In addition, the growth rate of the early half is
also positively correlated with the fault bar rate of the late half, indicating that the growth rate of the early
half positively influences the fault bar rate of the late half. This supports the view that the early growth is
most important in determining the overall fault rate.
The average growth rate of the octiles was substantially higher in the 7th
octile (Fig. 8). This higher
growth rate in the late growth phase could perhaps be explained by the fact that the tail feather used, the
R4 feather, is one of the longest tail feathers. These longer tail feathers keep growing when the shorter
ones have stopped, meaning that more resources are liberated to invest in the late part of their growth
since they are not competing for resources with the shorter feathers (Staffan Andersson, personal
communication, May 28, 2013).
In addition to average growth rate, increased tail length seems to add additional stress since the tail
length is positively correlated with fault bar rate. But, although the regression line was significant, it only
explained 10 % (Fig. 13) of the observed variance, less than half than what is explained by the regression
using average growth rate (Fig. 12). It seems therefore, that average growth rate is the more important
factor in determining the fault bar rate, i.e. increasing the feather growth rate is proportionally more
stressful than increasing the feather length by the same proportion.
It is also interesting to note that the fault bar score (i.e. absolute amount of faults) was significantly
higher in the 7th
than in the 1st octile, even when standardized for the higher growth rate in the 7
th (Fig. 9).
That the fault bar score is substantially greater in the late half of the feather could be due to a decline in
body condition towards the end of tail growth caused by the physiological costs of tail growth, as was
found to be the case in the Jackson’s widowbird (Andersson 1994).
Is male tail length an indicator of body condition?
Figure 10 shows a positive correlation between body condition and final tail length, as was found in a
study of the closely related species Jackson’s widowbird, E. jacksoni (Andersson 1994). This might
indicate that in order for males to grow a long tail they need to be in good condition. However, the low
(0.09) R2 indicates that this relationship may not be that reliable, i.e. some males with low body condition
values still manage to grow long tails and some males with high body condition values only manage to
grow short tails. Because the body condition values are dependent on mass measurements taken at only
one point in time, maybe the males who had low body condition and long tails at the time of
measurement, had low body condition precisely because they had invested a large portion of their prior
condition into growing large tails. This seems unlikely though, since body condition does not seem to
correlate with the fault bar rate (i.e. stress), which should be the case if the long tailed males had invested
a large portion of their prior condition on growing long tails (i.e. they should show signs of stress at the
time of measurement due to having lost so much of their prior condition). Thus, it seems likely that,
although the relationship is far from perfect, tail length is at least to some extent representative of male
body condition. If this is true, then this suggests that tail length may serve as a male quality advertisement
in female mate choice.
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Future prospects
Regarding the fault bar measurements, it would be recommended for future studies using fault bars as an
indicator of physiological stress to use a classification of fault bars that not only takes fault bar length (as
this study does) but also fault bar intensity or width, into account, and see whether this affects the
resulting correlations and comparisons. Furthermore, it would be interesting to see more studies on the
effect of growth rate on the physiological costs of feather growth. This has been done for at least one
other widowbird species, namely the Jackson’s widowbird, E. jacksoni (Andersson 1994), with the same
result as in this study; i.e. fault rate increased with growth rate.
To be able to tell if male tail length really functions as a male quality advertisement in female choice in
the Buff-shouldered widowbird, further studies are required. For example the tail feather length could be
experimentally manipulated to see whether longer or shorter tail length affects female mate choice, as has
been done for the Long-tailed widowbird, E. progne (Andersson 1982) and Jackson’s widowbird
(Andersson 1992).
Because the body condition measure used in this study is, as previously stated, dependent on mass
measurements taken at only one point in time they might not be very representative of the males’ real
body condition. Thus, it would be desirable to see future studies that use a body condition measure that is
based on measurements taken at several points in time, and see if they get similar results.
Conclusions
Tail ornamentation seems to be costly to produce, since a positive relationship was found between tail
length and fault rate. Furthermore, increased growth rate seems to contribute even more to the cost of
producing tail feathers than increased tail length, particularly increased growth rate in the early phase of
feather formation. The fault bar score (a measure of the absolute amount of fault bars) was greater in the
late (last formed) half of the feather than in the early. Final tail length was weakly but significantly
positively related to body condition, suggesting that tail length may serve as a male quality advertisement
in the context of female mate choice.
Acknowledgements I want to thank my supervisor at the University of Gothenburg, Professor Staffan Andersson, who
introduced me to this study species and has helped me throughout the entire project with everything from
statistics and interpreting the results to critical review of the report. I would also like to thank Calum
Ninnes, one of Staffan’s PhD students at the University of Gothenburg, for supplying me with the feather
samples and helping me out with the measurements.
13
References Articles and literature Andersson, M. (1982). "Female choice selects for extreme tail length in a widowbird." Nature
299(5886): 818-820.
Andersson, S. (1992). "Female preference for long tails in lekking Jackson's widowbirds:
experimental evidence." Animal Behaviour 43(3): 379-388.
Andersson, S. (1994). "Costs of Sexual Advertising in the Lekking Jackson's Widowbird." The
Condor 96(1): 1-10.
Fisher, R. A. (1930). The Genetical Theory of Natural Selection. Clarendon Press, Oxford.
Fry, C. H., Keith, S (2004). The Birds of Africa Vol. VII., Christopher Helm, London.
Grubb, T. C. (1989). "Ptilochronology: Feather Growth Bars As Indicators of Nutritional Status."
Auk 106(2): 314-320.
James R. King, M. E. M. (1984). "Fault Bars in the Feathers of White-Crowned Sparrows: Dietary
Deficiency or Stress of Captivity and Handling?" Auk 101(1): 168-169.
Riddle, O. (1908). "The Genesis of Fault-Bars in Feathers and the Cause of Alternation of Light
and Dark Fundamental Bars." Biological Bulletin 14(6): 328-370.
Sarah R Pryke, S. A. (2003). "Carotenoid-based epaulettes reveal male competitive ability:
experiments with resident and floater red-shouldered widowbirds." Animal Behaviour 66(2): 217-
224.
Sarah R. Pryke, S. A. (2001). "SEXUAL SELECTION OF MULTIPLE HANDICAPS IN THE
RED-COLLARED WIDOWBIRD: FEMALE CHOICE OF TAIL LENGTH BUT NOT
CAROTENOID DISPLAY." Evolution 55(7): 1452-1463.
Sarah R. Pryke, S. A. (2002). "A generalized female bias for long tails in a short–tailed
widowbird." Philosophical Transactions of the Royal Society B: Biological Sciences 269(1505 ):
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Web sites
BirdLife (2013). "BirdLife Intenational." Retrieved 24/5, 2013, from http://www.birdlife.org
GoogleMapMaker (2013). "Iringa region, Tanzania." Retrieved 24/5, 2013, from http://goo.gl/Sq7dj
14
Appendix 1. Fault bar definitions:
Class 1: If the fault bar was found on only one side of the rachis and it went through less than half the
way across the barbs of one side of the rachis, or if found on both sides of the rachis and the sum of the
length of the fault bar fragments on both sides was less than half the way across the barbs of one side of
the rachis, then it fell into this class.
Class 2: If the fault bar was found on only one side of the rachis and it went through at least half the way
across the barbs of one side of the rachis or if found on both sides of the rachis and the sum of the length
of the fault bar fragments on both sides was equal to or greater than half way across the barbs of one side
of the rachis, but less than half way across on both sides (i.e. less than 1 1/2 way across the whole feather
width), then it fell into this class.
Class 3: If the fault bar went all the way across the whole feather width or if the sum of the fault bar
fragments on both sides was greater than half way across on both sides (i.e. more than 1 1/2 way across
the whole feather width), then it fell into this class.