The Auk 125(4):803–808, 2008
 The American Ornithologists’ Union, 2008��
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Printed in USA.
Effects of Specimen Age on Plumage Color
J essica K. A rmenta ,1 P eter O. D unn,
and
L inda A. Whit tingham
Department of Biological Sciences, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, USA
Abstract.—�������
Museum specimens
���������� are
���� valuable
��������� for
���� studies
�������� of
��� plumage
�������� color
������ in
��� birds,
������� but
���� feather
�������� color
����������
may fade
����� over
����� time
����� and
���� not
����
accurately reflect the colors of live birds. In particular, it has been suggested that ultraviolet (UV) color may be more susceptible to
degradation than human-visible colors. We used a reflectance spectrophotometer to measure feather color in five species of passerines
for which museum specimens were collected consistently over the past 100 years. We found that the feather colors of museum specimens
collected within the past 50 years were related closely to the feather colors of live birds. In fact, over a wide range of different colors, we
found little change in color measurements for recent (<50 years old) specimens. Furthermore, UV color was not affected more severely
by fading than human-visible colors when we confined our analyses to recent specimens. These results provide strong support for the
continued use of museum specimens to examine coloration in birds, provided that the specimens were collected relatively recently.
Received 5 January 2007, accepted 25 February 2008.
Key words: �������
color, fading,
�������� �������
museum �����������
specimens, ������������
reflectance ��������������
spectrometry, �������������
ultraviolet.
Efectos de la Edad de los Especímenes sobre el Color del Plumaje
Resumen.— ����
Los especímenes
������������ de
���������
museo son
���� valiosos
��������� para
����� estudiar
��������� el
��� color
������ del
���� plumaje
�������� en
��� las
���� aves,
������ pero
����� el
��� color
������ de
��� las
���� plumas
������� podría
�������
perderse a lo largo del tiempo y no reflejar los colores de las aves vivas. Específicamente, se ha sugerido que el color ultravioleta (UV)
podría ser más susceptible a degradarse que los colores visibles por los humanos. En este estudio, empleamos un espectrofotómetro de
reflectancia para medir el color de las plumas en cuatro especies de aves paserinas para las cuales se habían coleccionado especímenes
constantemente durante los últimos 100 años. Encontramos que los colores de las plumas de los especímenes de museo coleccionados en
los últimos 50 años se relacionaron cercanamente con los colores de las plumas de aves vivas. De hecho, a través de un amplio espectro
de colores, encontramos pocos cambios en las medidas de color para especímenes recientes (coleccionados hace menos de 50 años).
Además, el color UV no fue afectado más severamente que los colores visibles a los humanos al limitar nuestros análisis a especímenes
recientes. Estos resultados apoyan fuertemente el uso continuado de especímenes de museo para examinar la coloración de las aves si
los especímenes han sido coleccionados en tiempos relativamente recientes.
Museum study skins are widely used to assess plumage color
(e.g., Schmitz-Ornés 2006) and to study the evolution of color
traits and sexual dichromatism in birds (e.g., Owens and Hartley 1998, Dunn et al. 2001). However, feather color can be susceptible to fading over time, so recent studies have expressed concern
over the use of museum specimens (Winker 1997, McNett and
Marchetti 2005, Maley and Winker 2007). The measurement of
color has recently been improved by the use of reflectance spectrophotometers, because these instruments provide an objective
measurement of color and are able to measure ultraviolet (UV)
wavelengths invisible to humans (Bennett and Cuthill 1994). Almost all birds can see in the UV wavelengths (Burkhardt 1989),
and UV-reflecting plumage appears to be quite common among
birds (Eaton and Lanyon 2003). A recent study (McNett and
1
Marchetti 2005) found that UV coloration may be even more susceptible to fading in specimens than the colors humans can see.
Thus, it is not clear whether museum specimens accurately represent the color patterns and variation seen in live birds.
It is also possible that certain types of feather colors differ in
their susceptibility to fading. Feather color is generally produced
either by a pigment or the microstructure of the feather (McGraw
2006a, b; Prum 2006). The two most common pigments are carotenoids, which are responsible for most bright yellow, orange,
and red colors (McGraw 2006a), and melanins, which are responsible for most brown and black colors (McGraw 2006b). UV, white,
and blue are common structural colors (Prum 2006). McNett and
Marchetti (2005) suggested that carotenoid-based and UV colors
were more susceptible to fading than others.
Present address: �����������
Department ���
of ���������
Biology, ����������
Lone Star ����������������
College-CyFair, ���������
Cypress, ������
Texas �������
77433, �������������
USA. E-mail: [email protected]
The Auk, Vol. 125, Number 4, pages 803–808. ISSN 0004-8038, electronic ����������������
ISSN������������
1938-4254.  2008 by The American Ornithologists’ Union. All rights reserved. Please direct
all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals.
com/reprintInfo.asp.����������������������������
DOI: 10.1525/auk.2008.07006
— 803 —
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804
— Armenta, Dunn,
and
McNett and Marchetti (2005) measured the plumage colors
of 10 species of wood warblers (family Parulidae); however, they
were able to compare recently collected specimens with older
specimens (collected before 1935) for only 2 of these 10 species.
In the present study, we measured feather color of five species of
passerines for which we had series of specimens that were collected consistently over the past century. We examined the effect
of specimen age on feather colors over the 100-year period to determine whether some or all museum skins, regardless of age, are
suitable for studies of plumage color variation. Researchers can
choose to avoid using specimens that appear to the human eye to
be faded, but they may not be able to avoid unseen fading in the UV
range. Therefore, we also examined whether the effect of specimen
age on feather color was more severe in UV wavelengths than in
human-visible wavelengths.
M ethods
Data collection.—In November 2005 and August 2006, at the Field
Museum of Natural History in Chicago, Illinois, we measured spectral reflectance of plumage colors from 147 specimens of five species
of passerines (Table 1). The five species sampled were well represented in the collection and had been collected consistently over the
past 100 years. We selected these five species to represent commonly
found colors for inclusion in our study (Table 1). We limited our sampling to adult male specimens that were not visibly molting. We included only specimens whose feathers were not worn or dirty, but we
did not avoid specimens that appeared to be faded. Most specimens
were collected from midwestern states, and within each species most
specimens came from the same state or a neighboring state. Most of
the specimens were collected during the breeding season (late spring
to summer). Specimen collection dates ranged from 1892 to 2003.
In addition, we measured plumage reflectance of three live
Tree Swallows (Tachycineta bicolor) and 13 Common Yellowthroats (Geothlypis trichas) caught at the University of WisconsinMilwaukee (UWM) Field Station in Saukville, Wisconsin (30 May
to 5 June 2006). Birds were mist netted (Common Yellowthroats)
Whittingham —Auk , Vol . 125
or caught in nest boxes (Tree Swallows) and released after measurement. These species were chosen for live measurements because they were readily available at the UWM Field Station,
whereas the other three species were unavailable.
All reflectance measurements of plumage color were made
with an Ocean Optics USB2000 spectrophotometer and a PX-2 Xenon light source (Ocean Optics, Dunedin, Florida) and calibrated
against a WS-1 white standard, which reflects >98% of light from
250- to 1,500-nm wavelengths. A black rubber test-tube stopper
mounted on the end of the probe held the probe at a 90° angle to
the feathers and kept it at a fixed distance from the feathers. Reflectance measurements were made from 320 to 700 nm, because this
spectrum encompasses the bird-visible spectrum (Burkhardt 1989).
Representative color patches were selected for each species (Table 1),
and each color patch was measured five times per specimen.
Color calculations.—Each reflectance measurement was
transformed into variables of hue, saturation, and brightness
using the program SPECTRE (see Acknowledgments). All calculations were performed twice, once for the UV spectrum (320–
400 nm) and once for the human-visible spectrum (400–700 nm).
SPECTRE calculates brightness as the amount of light reflected
by the sample in relation to the amount of light reflected by the
white standard. The program calculates saturation and hue using
the segment classification method of Endler (1990). McNett and
Marchetti (2005) used similar methods for estimating color. Segment classification is advantageous in this type of study, because
it describes the shape of the reflectance curve and can be applied
without specific information about the visual system of the animal
(Endler 1990). This method divides the human-visible spectrum
(400–700 nm) into four equal regions that are approximately the
violet–blue, green, yellow–orange, and red wavelengths. For analyses of the UV spectrum, we restricted the segments to 320–400
nm; thus, each of the four equal segments covered 20 nm. Saturation is calculated as a point in two-dimensional space based on
the relative reflectance of each segment (Endler 1990). For example,
in the human-visible range, one axis is defined by the relative difference in reflectance between the red and green segments. The other
Table 1. Mixed-model ANOVA testing the effect of specimen age, spectrum (UV or human-visible), and their interaction on brightness, saturation,
and hue of avian plumage in five species. Separate analyses were conducted for brightness, saturation, and hue. For brevity, the effect of spectrum
(UV or human-visible) is not shown (see text). P values are shown, with significant values in bold (P < 0.017 after Bonferroni adjustment). Results of
the independent variable spectrum are omitted for brevity. P values with an asterisk remained significant when analyses were restricted to recent (<50
years) specimens, and a double asterisk indicates P values that became significant when analyses were restricted to recent specimens.
Species
n
Color
Body
region
Eastern Bluebird
24
Black-throated
Blue Warbler
Northern Cardinal
Common Yellowthroat
26
41
50
Tree Swallow
22
Rust
Blue
Blue-gray
White
Red
Yellow
Black
Brown
Green
White
Breast
Back
Back
Belly
Breast
Throat
Cheek
Back
Back
Belly
05_Armenta_07-006.indd 804
Spectrum*age interaction
Age
Brightness
Saturation
Hue
0.99
0.96
0.04
<0.01
<0.001
0.23
0.81
0.99
0.57
<0.001*
0.051
0.22
<0.01
<0.001*
0.76
0.02
0.20
0.30
0.001
0.06
0.55
0.18
0.26
0.99
<0.01
<0.001*
0.93
0.04
0.99
0.30
Brightness
0.32
0.02
<0.01
<0.01
0.90
0.40
0.77
0.04
0.21
0.25
Saturation
Hue
0.23
<0.01
<0.001
0.06 **
0.08
0.02
0.15
0.85
0.47
0.45
0.66 **
0.06
0.74
<0.001
<0.01
<0.001*
0.19**
<0.01
0.30
<0.01
10/15/08 5:04:24 PM
O ctober 2008
— Specimen Age
and
Color —
805
Fig. 1. Brightness of four colors of museum specimens over time. Closed circles and thick lines represent human-visible brightness (400–700 nm),
whereas open circles and thin lines represent brightness in the UV range (320–400 nm). Brightness does not change with age in green (A), brown (B),
or yellow (D), but decreases with age in red (C) (see Table 1 for P values).
axis is defined by the relative difference in reflectance between the
yellow–orange and violet–blue segments. Saturation is calculated
as the Euclidean distance between one point on the red–green
axis and the other point on the yellow–orange to violet–blue axis
(equation 16 in Endler 1990). As the relative difference between reflectance of the segments increases, so does saturation, such that
red would have a higher saturation than pink. Hue, the portion
of the electromagnetic spectrum where the sample shows maximum light reflectance, is the attribute that is commonly thought
of as “color,” such as red or blue (Bennett et al. 1994). Hue is an
angular variable, such that values of hue rotate 360° around the
axes defined by the four segments. It is calculated as the arcsine
of the product of the difference between the reflectance of the
yellow–orange and violet–blue segments divided by the saturation
(equation 17 in Endler 1990). Within each spectrum, the values for
brightness, saturation, and hue for the five repeated samples were
averaged for each body region of each bird.
Statistics.—We performed a mixed-model analysis of variance
(ANOVA) on each color patch of each species for brightness, saturation, and hue. Brightness, saturation, or hue was included as the
dependent variable in each analysis, and specimen age (the number of years between the specimen collection date and the date of
05_Armenta_07-006.indd 805
sampling) and the spectrum (UV or human-visible) were included
as independent variables. Specimen age was treated as a continuous
variable and spectrum as a categorical variable. Because each specimen was effectively sampled twice in this analysis, once as a UV
measurement and once as a human-visible measurement, the specimen identification number was included as a random effect.
We were also interested in determining whether more recent
specimens would be more suitable for use in plumage color studies and, thus, we categorized all specimens as either “recent” (<50
years since collection or live) or “old” (>50 years since collection).
Fifty years was chosen as the cutoff age after we examined changes
in color variables over time, and also because a gap in the collection record for most species occurred ~50 years ago (Fig. 1). Recent specimens included 6 of 24 Eastern Bluebirds (Sialia sialis),
14 of 26 Black-throated Blue Warblers (Dendroica caerulescens),
23 of 41 Northern Cardinals (Cardinalis cardinalis), 38 of 50 Common Yellowthroats, and 10 of 22 Tree Swallows. Separate mixedmodel ANOVAs were performed for recent specimens to determine
whether significant age and interaction effects (spectrum*age) were
attributable to the inclusion of older specimens.
Finally, not all body regions of a specimen are affected equally
by normal handling and storage conditions (e.g., specimens are
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— Armenta, Dunn,
and
Whittingham —Auk , Vol . 125
usually stored on their backs with their bellies up), and so we performed mixed-model ANOVAs to examine the relationship between body region and specimen age. All museum specimens
and live birds were included in these analyses, with brightness,
saturation, or hue as the dependent variable. Specimen age and
body region were included as independent variables, with specimen number and species included as random effects. In all mixedmodel ANOVAs, we used a Bonferroni adjustment of our level
of significance (to 0.017), because the same data were used three
times to produce estimates of hue, saturation, and brightness.
R esults
There was no effect of specimen age on rust, blue, or black plumage colors, but other colors showed variable effects of age (Table 1).
Blue-gray, white, and red showed a significant decrease in brightness with age (Table 1 and Fig. 1). Both recent and older specimens
were variable in brightness (Fig. 1), so age was not a reliable predictor of brightness. Saturation and hue also decreased over time
in some colors but not in others. For example, the saturation of
yellow and green decreased with specimen age, but the saturation of white increased with age in the Black-throated Blue Warbler, though not as strongly in the Tree Swallow (P = 0.06). In the
case of blue-gray, saturation decreased with specimen age in the
human-visible spectrum but increased with specimen age in the
UV spectrum. Only yellow showed an increase in hue with specimen age. Interestingly, a significant decrease in brightness over
time did not necessarily result in a significant change in either saturation or hue, or vice versa (Table 1). Differences between the UV
and human-visible sections of the spectrum were almost always
significant (spectrum effect: all P < 0.02); the human-visible spectrum typically had higher values of brightness, saturation, and
hue than the UV spectrum.
We found a significant interaction between spectrum (UV and
human-visible) and specimen age for the brightness of some colors but not others (Table 1), which indicates that fading occurred
at different rates in different parts of the spectrum. In two colors,
the blue of the Eastern Bluebird and the brown of the Common
Yellowthroat, UV brightness decreased, whereas human-visible
brightness increased with specimen age (Fig. 1). For two other
colors, the blue-gray and white of the Black-throated Blue Warbler, brightness decreased with age, and the decrease was faster
in the UV spectrum than in the human-visible spectrum (Fig. 2).
Saturation and hue are not necessarily predicted to change in the
same direction for both the UV and human-visible spectra, because they depend on the shape of the reflectance curves within
each spectrum (UV and human-visible), and the shape of the
reflectance curve is usually different between the two spectra.
Therefore, we did not attempt to interpret the interaction terms
for saturation and hue.
By contrast, when only recent specimens were included in the
mixed-model ANOVAs, most colors showed no significant effect
of age on brightness, saturation, or hue. Indeed, only the brightness of the white belly of Tree Swallows (F = 10.1, df = 1 and 10, P =
0.02), saturation of the white breast of Black-throated Blue Warblers (F = 12.4, df = 1 and 14, P < 0.01), and the hue of the yellow
throat of Common Yellowthroats (F = 7.3, df = 1 and 38, P = 0.01)
were influenced by specimen age in recent birds.
05_Armenta_07-006.indd 806
Fig. 2. Reflectance from the white belly of Black-throated Blue Warblers.
Lines are averages from recent (<50 years old, thin, n = 14) and old (>50 years
old, thick, n = 12) specimens. Differences between recent and older specimens were not tested statistically but are shown here for illustrative purposes.
The UV range is 320–400 nm, and the human-visible range is 400–700 nm.
None of the interaction terms were significant for brightness.
Therefore, the brightness of plumage was generally not fading faster
at UV than at human-visible wavelengths when the analysis was restricted to recent (<50 years old) specimens (Table 1). The decrease in
plumage reflectance of the yellow throat of Common Yellowthroats
was not greater in the UV than in the human-visible wavelengths
(comparing live and all museum specimens), and the reflectance of
the yellow was almost identical between live birds and recent specimens (<2% difference; Fig. 3). This difference between live and
Fig. 3. Reflectance from the yellow breast of Common Yellowthroats.
Lines are averages from live (dashed, n = 13), recent (<50 years old, thin,
n = 25) and old (>50 years old, thick, n = 11) specimens. Differences between live birds, recent specimens, and older specimens were not tested
statistically but are shown here for illustrative purposes. The UV range is
320–400 nm, and the human-visible range is 400–700 nm.
10/15/08 5:04:29 PM
O ctober 2008
— Specimen Age
Table 2. Mixed-model ANOVA testing the effect of specimen age and
body region (back, belly, breast, cheek) on brightness, saturation, and hue
of five species. Separate analyses were performed with brightness, saturation, or hue as the dependent variable. Specimen identity and species
were included as random factors in each model (n = 163 specimens).
Variable or interaction
Age
Body region
Age*body region
Brightness
<0.001
<0.001
<0.001
Saturation
0.012
0.003
<0.001
Hue
0.015
<0.001
0.057
recent specimens disproportionately affected one portion of the
spectrum, however, so it was enough to cause a significant difference in hue between the two groups.
We found evidence that some body regions, regardless of
species, were affected more severely than others by specimen
age. In mixed models with body region and species as random
factors (Table 2), the interaction term between age and body region was significant for brightness (F = 15.8, df = 4 and 163, P <
0.001) and saturation (F = 6.0, df = 4 and 163, P < 0.001), but not
for hue (F = 2.3, df = 4 and 163, P = 0.057). The particular body region affected by age was not the same for brightness and saturation. For brightness, only measurements of the belly, which was
the brightest region, declined with specimen age, whereas the
brightness of the other body regions was relatively unaffected by
specimen age. For saturation, only measurements of the cheek
increased with age.
D iscussion
We were able to obtain measurements of both museum specimens
and live birds for two species, the Common Yellowthroat and
the Tree Swallow. For these two species, we found that museum
specimens collected within the past 50 years accurately reflected
feather colors of live birds. Time did not play a strong role in the
color variation found among recent (<50 years old) specimens
over a wide range of different colors; this effect may be attributable, in part, to small sample sizes. Furthermore, UV wavelengths
were not affected more severely by fading than human-visible
wavelengths, when we confined our analyses to recent specimens.
By contrast, several colors differed with specimen age in old (>50
years old) specimens. If we categorize colors by the mechanism
used to produce them, we found that structural (green, blue-gray),
melanin-based (brown), and carotenoid-based (yellow, red) colors
all differed with specimen age in old specimens. In recent specimens, some structural (white) and carotenoid-based (yellow) colors changed with age, though the changes were less than those
observed in old specimens. Therefore, change in feather colors
over time was not related to the color, including UV, or to the
mechanism that produced the color.
White appears to be the color most susceptible to changes over
time. The white of Tree Swallows decreased in brightness over time,
even in more recent specimens. Also, the white of Black-throated
Blue Warblers showed an increase in saturation over time. Saturation is very low in a pure white color, so changes in reflectance at any
wavelength will increase saturation (Fig. 2). Thus, specimens that
05_Armenta_07-006.indd 807
and
Color —
807
become dirtier over time, perhaps from handling, are likely to increase in saturation. Indeed, museum specimens with large patches
of white often looked “dirty” to us in comparison to live birds.
White may be most susceptible to change because it has the highest
brightness and overall reflectance of all colors.
We also found that body region influenced whether the color
faded over time. The belly was most severely affected by fading.
This may be attributable to storage conditions, because specimens
are usually stored on their backs, with their bellies facing upward.
Alternatively, the fading may be a result of handling of specimens
over time, given that the belly is often touched when specimens are
handled. Plumage color in some other body regions, such as the
wings, may also be affected by handling; however, we did not measure wing color, so further study is warranted.
Like McNett and Marchetti (2005), we found that most colors were affected by age in older specimens. However, when we
restricted our analyses to recent specimens, we found few effects
of age on feather color. It is possible that the effects of time in recent specimens were masked by small sample sizes, a slow rate of
change, and high individual variance in specimen color. Such a
slow rate of change in color, however, suggests that researchers
could use recent specimens with some confidence. In particular,
for researchers interested in UV coloration, we did not find that
colors fade faster in the UV than in human-visible wavelengths, as
was reported by McNett and Marchetti (2005).
McNett and Marchetti (2005) suggested that the greater decrease in brightness in the UV than in the human-visible wavelengths would result in changes in hue and saturation, because hue
and saturation are calculated on the basis of the relative brightness
of the four color segments (Endler 1990). In all the colors that we
measured, however, we found that a change in brightness was not
necessarily related to a change in hue or saturation. The difference
between our results and those of the previous study are probably attributable to our use of a series of more recent museum specimens.
McNett and Marchetti (2005) compared live birds with specimens
that were mostly collected before 1935. Our use of more recent
specimens seems to eliminate most of the change in color associated with age that they observed. The reflectance of yellow from recent museum specimens almost exactly matched the reflectance of
live Common Yellowthroats (Fig. 3), in contrast to figure 3A in McNett and Marchetti (2005). Another possible explanation for the
difference in results between studies is that the specimens examined by McNett and Marchetti (2005) may have been prepared using a preservative that decreased UV brightness but did not affect
human-visible brightness. This type of accidental staining, which
would be invisible to the human eye, has been found in some museum specimens (Pohland and Mullen 2006). Unfortunately, information about whether a preservative was used on a specimen,
or about what type of preservative was used, is rarely available for
older specimens. Although preservatives were once commonly
used, most recent specimens are prepared without added preservatives (Winker 2000). It is also possible that specimen color could be
affected by washing during preparation, either through removal of
stains and dirt on the plumage or through an effect of detergent on
the reflectance of feathers. Unfortunately, preparators usually do
not record whether a specimen has been washed.
The present study strongly supports continued use of museum specimens in examining plumage coloration in birds,
10/15/08 5:04:30 PM
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— Armenta, Dunn,
provided that the specimens were collected recently. Results of
studies using older specimens may need to be interpreted with
caution, given that many colors are quite susceptible to changes
in hue, saturation, or brightness. Our results emphasize the importance of continued collecting, because many large comparative studies would not be feasible using only live birds.
Acknowledgments
We thank J. Bates, S. Hackett, and D. Willard at the Field Museum
of Natural History in Chicago, Illinois, for access to specimens.
J. Berges, K. Winker, K. Yasukawa, and two reviewers provided
helpful comments on the manuscript. This work was supported by
National Science Foundation (NSF) DEB-0215560 to P.O.D. and
L.A.W. and by an NSF graduate research fellowship to J.K.A. The
program SPECTRE is available at www.uwm.edu/~pdunn/Spectre/
Spectre.html.
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Associate Editor: K. Winker
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