Journal of
Comparative
Physiology A
J Comp Physiol A (1989) 164:787-796
Sensory,
.~.-.
~.~,%"
and
9 Springer-Verlag 1989
U V vision: a bird's eye view of feathers*
Dietrich Burkhardt
Institut ffir Zoologic, Universitfit Regensburg, D-8400 Regensburg, Federal Republic of Germany
Accepted November 9, 1988
Summary. The spectral reflectance of feathers was
measured in the range between 310 and 730 nm
by means of a diode array spectrometer. In many
feathers an ultraviolet (UV) reflection adds to the
reflection in the visible range which causes their
coloration as seen by man. The UV reflectance
is related to the presence of pigments in feathers
and to the arrangement of structures which influence the light reflecting properties. Feathers giving
strong UV reflection are called type A, without UV
reflection type B, and giving weak to medium UV
reflection type A/B. On the assumption that birds
are tetrachromates, colour vision in birds and their
possible chromaticity diagrams are discussed. If
red, green, blue and UV are primary colours, three
secondary colours are present in the daylight spectrum: yellow, blue-green, and violet-ultraviolet.
Three more secondary hues may originate from
mixing spectral lights: purple (red and violet),
'bird's purple' (red and UV), and 'green purple'
(green and UV). Some feathers with doublebanded reflectance curves will produce hues which
are not present in the daylight spectrum.
Introduction
Schiemenz (1924) was the first to discover ultraviolet (UV) vision in a vertebrate, the minnow, yet
evidence for UV vision in birds was presented
much later (Huth and Burkhardt 1972; Wright
1972). Since these first accounts on hummingbirds
and pigeons, UV sensitivity records have been collected for more than 30 bird species. 10 of the 16
families involved belonging to the Passeriformes
(compare Chen and Goldsmith 1986). However,
wavelength discrimination curves including the UV
* Dedicated to Johann Schwartzkopff on his 70th birthday
are rare (pigeon: Emmerton and Delius 1980;
hummingbirds: Goldsmith et al. 1981), most studies focusing on retinal mechanisms. At the receptor level the situation seems to be complex: several
types of cones and oil droplets of various colours
determine the spectral response. UV vision could
be mediated by retinal mechanisms with a peak
sensitivity at 400 nm (Graf and Norren 1974; Norren 1975), or a peak sensitivity at 370 nm (Chert
et al. 1984). Sensitivity peaks within the range visible to man occur at around 420, 450, 480, 510
and 570-600 nm, depending on the species investigated (see Chen and Goldsmith 1986; Jane and
Bowmaker 1988). In training experiments with the
Pekin nightingale (or red-billed Leiothrix) we have
just established four sensitivity peaks located at
380, 470, 530 and 620 nm, the sensitivity being
nearly five times higher in the UV than in the visible range (Burkhardt and Maier 1989). From these
results it seems likely that colour vision in birds
is at least tetrachromatic, yet nothing is known
about the chromaticity diagram including the UV
in any bird.
Only few papers deal with the biological significance of colour vision including the UV in birds
(reviews: Burkhardt 1983, 1988; Bowmaker, in
press). UV vision might play a role in homing
(Kreithen and Eisner 1978) and migration, in foraging for food (Burkhardt 1982), and in recognition of plumage colours (Burkhardt 1983). In contrast, in insects the significance of the UV in
visually guided behaviour has received a great deal
of attention (see Daumer 1958; Silberglied 1979;
Meyer-Rochow and Eguchi 1983; Schwind 1985;
Wehner 1987; Menzel 1987). Since we know that
in birds coloration plays an important role in intraand interspecific behaviour, it is surprising that
next to nothing is known about the reflectance
properties of bird's feathers in the UV range, the
788
available data being restricted to the spectral range
visible to man (see Voitkevich 1966; Dorst 1974;
Burrt 1986; Durrer 1986).
Our first attempt to fill this gap was to photograph some 'bird-relevant' objects in both the UV
and visible ranges. This approach is useful in the
field (Lutz 1924; Daumer 1958; Eisner et al. 1969;
Burkhardt 1982), but yields satisfactory results
only if the filters applied can be matched to the
absorption properties of the visual system. Therefore, a better approach is to record the spectral
reflectance of an object under question with a photometer. The goal of the present study was to take
advantage of recent developments in photometers
(Gerlinger and Schlemmer 1988) which have made
such measurements rapid and simple. I present
here spectral reflection data including data from
the UV range for a variety of feathers.
Material and methods
Single feathers of various birds, found in the field or in aviaries,
were collected. Some feathers were taken from birds killed by
accidents, some from mounted specimens in the department's
collection. Measuring was done without previous cleaning, although sometimes the barbs or barbules had to be rearranged.
Reflectance was recorded with a single-beam Zeiss MCS 230
photometer unit in 1 nm steps and a resolution of 2.5 nm. The
light source was a Zeiss CLX 111 Xenon unit, and the reference
of 100% reflectance was a MS 20 ceramics white standard. The
feathers were placed on black velvet in a Zeiss G K 111 goniometer; the angle of light incidence was 45 ~ and the reflectance
was measured at an angle of 0 ~ or 25 ~ The measuring light
spot had a diameter of about 2 ram. A pair of UV-transparent
achromatic lenses and light guides connected the goniometer
with the photometer and its light source; the convergence and
divergence angles of the light beams were 25 ~. For reflectance
bands due to interference this arrangement might slightly increase the half-width of reflectance bands and cause a minor
shift of the peak wavelength. The recorded reflectance values
were probably below the actual reflectance values of the
feathers. Two effects contribute to this. (1) The reflectance
values decrease if a feather is not placed exactly in the focal
plane. (2) If the barbs and barbules do not cover the measuring
spot entirely, part of the incident light is absorbed by the black
velvet background. However, neither effect distorts the shape
of the spectral reflectance curves or their maxima, minima and
slopes.
Some preliminary measurements of U V reflectance were
obtained by means of photography, using a calibrated grey
scale placed next to the object Schott filters (3 mm U G 1 plus
2 m m BG 38) were used as a U V filter. The transmission peak
of this filter combination was 65% at 365 nm; transmission
below 320 nm and above 400 nm was less than 1%o. Data obtained with this technique are marked with ~
in the Tables.
The examined feather samples are classified according to their
reflectance properties: type A, reflectance in both the visible
and the U V range; type B, reflectance in the visible range only;
type A/B, reflectance in the visible range combined with weak
to medium reflectance in the U V range. This classification
might be helpful at present, yet it should be emphasized that
all intermediate types of U V reflection may occur, and that
D. Burkhardt: UV-vision: a bird's eye view of feathers
there is variation between various spots of the same feather,
between feathers of the same bird, and between comparable
feathers of specimens of the same species.
Munsell notations of the feathers were taken by comparing
feathers with samples in the Munsell book of color (1976) in
diffuse daylight.
Results
Black, white and grey feathers
All black feathers investigated as yet, whether velvety or shiny, have shown no detectable reflectance, either throughout the spectral range visible
to man or in the UV (Table 1). Two examples of
reflectance types found in white feathers are given
in Fig. I. White of the type A is represented by
a feather of the snowy owl: the reflectance is high
both in the visible range and in the UV. In type A/
B white, represented by the reflectance of a white
pigeon feather in Fig. 1, reflectance is high in the
visible range but declines below 400 nm. Samples
of type A white and A/B white feathers are listed
in Table 1. So far, no type B white has been found.
Reflectance curves of grey feathers are lower
than those of white feathers, but otherwise similar:
either the reflectance curve is flat throughout the
visible and the UV (type A grey) or there is a drop
below 400 nm (type A/B grey). An example of the
type A grey is the grey parrot, while belly feathers
of the robin are of type A/B grey.
Red, chestnut and brownfeathers
In red feathers type A as well as type B reflectance
was found. For example, type A red is present in
(~
60
R
45
.,, ,...,..
.............................................................................
30
15
I
0
3O0
400
I
i
I
i
500
~
600
i
I
(nm) 700
Fig. 1. Spectral reflectances of white feathers. Continuous
curve: wing secondary of a snowy owl, Nyctea scandiaca, type
white A, high U V reflectance. Dotted curve: wing primary of
a pigeon, Colurnba livia, type A/B white, low U V reflectance.
Abscissa: wavelength in nm; ordinate: reflectance (R) in per
cent as related to the white standard. The same conventions
arc used in Figs. 2-10 throughout
789
D. Burkhardt: UV-vision: a bird's eye view of feathers
Table 1. Black, white and grey feathers
Colour
Origin of feather
Species
Remarks
Munsell notation
(hue/vahie/chroma)
Black,
no UV
Raven,
wing primary
Rook,
wing primary
Magpie,
wing primary
Blackbird,
breast, juvenile
Great hornbill,
wing primary
Snowy owl,
breast
Bullfinch,
belly
White wagtail,
belly
Herring gull,
breast
Naked-throated bellbird
Grey Parrot,
undertail coverts
Egyptian vulture,
wing
Pigeon,
wing primary
Griffon vulture,
wing primary
Heron,
breast
Mute swan,
wing primary
Great hornbill,
wing secondaries
Herring gull,
wing
Wheatear,
back
Nuthatch,
back, wing
Grey parrot
Robin,
belly
Pigeon,
wing primary
Corvus corax
Shiny"
N/l/-
Corvus frugilus
Shiny a
N/1 / --
Pica pica
Shiny, phot
Turdus merula
Velvety, phot
Buceros bicornis
Shiny, phot
Nyctea scandiaca
Velvety
Pyrrhula pyrrhula
Shiny, phot
Motacilla alba
Shiny, phot
Larus argentatus
Shiny, phot
Procnias nudicollis
Psittaeus erithacus
Phot
Velvety, phot
Neophron perenopterus
Phot
Columba livia
Shiny b
Gyps vulvus
Shiny
Ardea cmerea
Velvety
Cygnus olor
Shiny
Buceros bicornis
Shiny, phot
Larus argentatus
Phot
Oenanthe oenanthe
Phot
Sitta europaea
Phot
Psittacus erithacus
Erithacus rubecula
Phot
Columba Iivia
Shiny, phot u
White A,
high UV
White A/B,
low UV
Grey A,
high UV
Grey A/B,
low UV
N/9.5/-
N/9.5/-
N/9.5/-
N/4.25/--
" u v vision demonstrated for the genus
b UV vision demonstrated for the species
the scarlet ibis (Fig. 2); these feathers have a towpass filter p r o p e r t y in the visible range, the reflectance rising steeply above 580 nm, and there is an
additional reflectance b a n d in the U V at a r o u n d
350 nm. These feathers are listed in Table 2. Type B red is represented by red tail feathers o f the
great spotted w o o d p e c k e r (Fig. 2): While the rise
o f reflectance a r o u n d 600 n m is similar to the rise
in the type A red, the s e c o n d a r y reflectance p e a k
in the U V is missing.
Quite different is the type B reflectance curve
o f chestnut-cotoured feathers, such as in the taiI
o f the black redstart (Fig. 2). T h e r e is n o reflectance t h r o u g h o u t the U V and the short wavelengths in the visible range. A b o v e 500 n m the reflectance rises, first gradually, and then, in the region above 550 nm, nearly linearly. Chestnut
feathers are listed together with type A red and
type B red in Table 2.
M e d i u m and d a r k b r o w n feathers exhibit reflection curves similar to the chestnut feathers, yet
in general the reflectance is lower. A p a r t f r o m a
790
D. Burkhardt: UV-vision: a bird's eye view of feathers
25
50
(%)
E%]
40 -
20
R
R
~ "
30
15
20
10
10
0
I
300
I
400
I
I
I
I
I
I
500
),
600
[nrn]
700
0
I
300
Fig. 2. Spectral reflectances of red feathers. Upper, continuous
curve: wing primary of the scarlet ibis Edocimus ruber, type A
red, high UV reflectance, peaking at 370 nm. Middle, dotted
curve : tail feather of the great spotted woodpecker Dendrocopus
major, type B red, nearly no UV reflectance. Lower, solid curve:
tail feather of a female black redstart, Phoenicurus rubecula,
chestnut, type B, no UV reflectance
rise of reflectance above 600 nm, the reflectance
of whitish- and greyish-brown feathers resembles
that of grey feathers of type A or A/B. An example
of greyish-brown of type A is given in Fig. 3.
Yellow feathers
Most of the yellow feathers investigated have a
reflectance curve similar to type A red feathers,
namely a reflectance band in the UV below
I
I
400
I
I
I
I
I
500
h
600
~m)
700
Fig. 3. Greyish-brown wing primary of a goshawk Accipiter
gentilis. Type A greyish-brown, UV reflectance high
350 nm, almost no reflectance in the short range
of visible wavelengths, and a sharp rise of reflectance to a high level throughout the long wavelength range. The rise occurs between 450 and
550 nm, thus causing hues ranging from greenishyellow, through vivid yellow, to orange. Representative samples are yellow parts of feathers of a
budgerigar and a white-fronted amazone (Figs. 4,
9). This type A yellow in the Psitticaformes resembles that of Passeriformes, yellow feathers of which
may have a considerable UV reflectance as was
shown mostly by filter photography. Obviously,
these passeriform feathers are also type A yellow.
Yellow-orange feathers of the red-billed Leiothrix
Table 2, Red, chestnut and brown feathers
Colour
Origin of feather
Species
Red A,
high UV
Scarlet ibis,
wing primary
Greater flamingo,
wing primary
Grey parrot,
red tail feather
Chestnut-fronted macaw,
forehead
White-fronted amazone,
wing secondary
Bullfinch, male,
breast
Scarlet ara
Great spotted woodpecker,
tail
Robin,
throat
Black redstart
Pheasant,
tail
Goldfinch,
back
Ruddy shelduck
Edocimus ruber
Red B,
no UV
Chestnut,
no UV
Brown
(similar to chestnut),
no UV
a UV vision demonstrated for the genus
Remarks
MunselI notation
(hue/value/chroma)
7.5 R/5/6
Phoenicopterus ruber
Phot
Psittacus erithacus
Phot
7.5 R/5/10
Ara severa
Amazona albifrons
Pyrrhula pyrrhula
7.5 R/8.5/12
Phot
Ara macao
Dendrocopusmajor
8.75 R/5/12
8.75 R/4/M
Erythacus rubecula
7.5 YR/5/8
Phoenicurus ochruros
Phasianus ochruros
7.5 YR/5/6
7.5 YR/5/6
Carduelis carduelis
Phot a
Casarcaferruginea
Shiny
7.5 YR/5/6
D. Burkhardt: UV-vision: a bird's eye view of feathers
791
25
5O
(%)
20
R
15
10
5
-
0
300
I
400
I
I
500
I
)~
I
600
I
i
[nm] 700
0
I
300
Fig. 4. Spectral reflectance of a yellowish-white feather of a
budgerigar Melopsittacus undulatus (dotted line), type A yellow,
UV reflectance high. While the yellow feather reflects the long
wavelength range, for comparison the second (continuous)
curve gives the reflectance of a blue feather of the same bird
with a reflectance band in the short wavelength range
I
I
400
I
I
I
I
I
500
~
600
(am)
700
Fig. 5. Spectral reflectance of the velvety-green part of a feather
of a white-fronted amazone Amazona albifrons. Type A green,
high UV reflectance
Table 3. Yellow feathers
Colour
Origin of feather
Species
Yellow A,
high UV
Rose-ringed parakeet,
breast
White-fronted amazone,
wing secondary
Budgerigar
Greenfinch,
wing primary
Waxwing,
tail
Blue-headed wagtail,
breast
Great hornbill,
neck
Psittacula eupatria
10 Y/7/8
Amazona albifrons
5 Y/8_5/~2
Yellow B,
no UV
Remarks
Melopsittacus undulatus
Carduelis chloris
Phot, mat&
Bombycilla garrulus
Phot
Motacillaflava
Phot
Buceros bicornis
Phot
Munsell notation
(hue/value/chroma)
2.5 GY/9/4.
a UV vision demonstrated for the genus
lack a UV reflection, as do some pale yellow and
grey-yellow feathers (shown by filter photography
in the great hornbill). Thus it seems justified to
subdivide yellow feathers into a type A yellow and
a type B yellow (Table 3).
Greenfeathers
To date, four types of reflectance curves have been
found in green feathers. Green of the type A was
found in some green feathers of parrots, an example being the white-fronted amazone (Fig. 5). The
reflectance curve partly resembles that of the type A yellow: there is an UV reflectance band below
350 nm, and the reflectance is low in short wavelengths .of the visible range and rises steeply above
500 rim, peaking at about 540 nm. However, in the
region between 550 and 600 nm the reflectance declines.
Green of types A/B and B seems to be rare,
yet feathers of the greenfinch's back, olive coloured, are lacking reflectance in both the UV and
adjacent short visible wavelength ranges. A gradual rise above 450 to 500 nm leads to a flattened
peak at around 560 nln and a decline with longer
wavelengths.
While dull or velvety green feathers (types A
and B) have a fairly wide spectral range of reflectance, iridescent green feathers either have only
a single and sharp reflectance peak in the green
region (type B interference green) or there may be
secondary peaks in the UV to violet region (type A
interference green). An example of strong additional UV to violet reflection is the iridescent green
part of some feathers of the Indian peafowl
(Fig. 6), while using filter-photography it was
found that the iridescent green feathers of the quetzal (Pharomachrus mocino) and a hummingbird
792
D. Burkhardt: UV-vision: a bird's eye view of feathers
25
75
(%)
(%)
20
60
R
R
15
45
30
5
..
9
0
".-..........
I
I
400
300
I
15
"...
............. ...""
..., ......
I
500
I
~
I
600
I
(nm)
I
700
Fig. 6. Spectral reflectances of iridescent green parts of feathers
of the Indian peafowl Pavo cristatus. In different parts of the
plumage iridescent green colours can be found with strong or
weak additional reflectance in the short wavelength region.
Continuous curve: type A interference green, secondary peaks
at 410 and 330 nm. Dotted curve: type B interference green,
hardly any reflectance in the U V
0
I
300
I
400
I
I
500
I
~
I
600
I
I
(nm) 700
Fig. 8. Spectral reflectance of a shiny violet feather found in
the Malay rainforest, probably from the Asian fairy bluebird
Irena puella
50
40
R
50
30
(%)
:.'"..'~...i'i~:.,,.:..................................
4O
R
30
10
20
0
.:-!
I
300
10
I
0
300
I
400
]
I
I
I
500
X
600
I
I
(nm) 700
Fig. 7. Spectral reflectance of the velvety-blue parts of a feather
of the jay Garrulus glandarius, type A blue
(Lesbia sparganura) have weak or no UV reflectance.
Blue and violet feathers
An example of a bright blue although velvety
feather is found in the jay (Garrulus glandarius).
The blue parts of these feathers exhibit a broad
reflectance band spanning the UV to green region
of the spectrum with the peak at 380 nm (Fig. 7).
The reflectance of feathers from the blue-and-yellow macaw (Ara ararauna) is similar, while blue
feathers of the budgerigar (Melopsittacus undulatus) show a wider reflectance band with a plateau
between 350 and 450 nm, declining above and below these wavelengths (Fig. 4). A shiny blue-violet
feather found in the Malay rain forest is probably
from an Asian fairy bluebird (Irena puella). With
I
400
I
I
I
l
500
k
600
I
I
(nm) 700
Fig. 9. Spectral reflectances of different parts of a feather of
the white-fronted amazone Amazona albifrons. Continuous
curve: red part near the base of the feather, type A red. Dotted
curve: yellow part, middle of the feather, type A yellow. Densely dotted curve: green part, near the tip, type A green. Note
that the reflectance of the green part is the lowest above 620 nm
and highest below 400 nm as compared with the other curves
its peak around 360 nm, the reflectance shows a
steep decline towards 500 nm and then declines
more gradually towards the red end of the spectrum (Fig. 8). Obviously, the strongest reflectance
is in the UV and the feather of this bird might
be called UV-violet rather than blue-violet. From
filter photography it was deduced that the violet
patches on the budgerigars's head have a reflectance curve similar to that shown for the UV-violet
feather of Fig. 8.
Multicoloured feathers
Many feathers have striking multicoloured patterns. An example is a yellow, green and red spotted feather of a white-fronted amazone (Fig. 9).
The red, yellow, and green are all of the respective
A types, that is, they have a considerable amount
D. B u r k h a r d t : UV-vision : a bird's eye view of feathers
25
20
R
15
10
0
I
300
I
400
I
I
I
I
500
X
600
I
I
(nm) 700
Fig. 10. Spectral reflectances of the eye of a male Indian peafowl
tail feather. C o n t i n u o u s curve : bluish-green ring, type A interference green. D o t t e d curve: blue ring a r o u n d the centre, no
secondary peak in the UV, type B interference blue. Densely
dotted curve, outer bronze-colouring ring, secondary peaks at
530, 450 a n d 350 n m
of UV reflectance. In the visible range, however,
the spectral reflectance curves differ significantly
between these colours: the red and the yellow
feather spots have plateau reflectance curves with
cutoff slopes at different wavelengths (red, 600 nm;
yellow, 500 nm). The green spots have a similar
cutoff slope to the yellow ones but have a stronger
reflectance, i.e. a marked peak, between 500 and
550 nm and a weaker reflectance in the long wavelength range compared with both yellow and red
spots.
In Fig. 10 various parts around the eye of a
male Indian peafowl tail feather have been studied.
There were between one and three reflectance
bands. Depending on colour, peaks occurred from
the red part of the spectrum down into the UV
region. For example, in the bronze-coloured ring
a major peak was found at 630 nm, a second peak
at 520 nm, a third at 450 nm, and finally a slight
rise below 390 nm. The brilliant blue region had
a single reflectance peak at 470 nm, and the green
part had reflectance peaks at 550 and 400 nm and
a minor band below 330 nm. Finally, the dark violet core showed a weak reflectance band at 390 nm
and a slight rise of reflectance below 350 nm.
Discussion
Origin of plumage colours
The spectral reflectances of the samples showed
considerable differences in the UV range. A few
remarks on the origin of plumage colours may be
useful for understanding the possible mechanisms
causing different UV reflectances in various
feathers.
793
While in feathers of the A type the reflectance
between 300 and 400 nm was fairly high, type B
and type A/B feathers either showed no reflectance
in the UV or a marked decline below 400 nm. It
seems reasonable to relate the lack or presence of
UV reflectance chiefly to the melanin content of
the feathers. Light absorbance by melanin pigments is known to increase progressively with the
reciprocal of the wavelength down to 300 nm
(Needham 1974). Melanin-caused colorations
range from pale yellow and chestnut, to brown
and black. In many feathers the melanin content
changes towards the base (Dorst 1974). Melanincontaining feathers are not expected to reflect
much UV.
In contrast to the melanin pigments, some carotenoids do not absorb UV. They absorb strongly
in the blue part of the spectrum and have a steep
decline of absorbance in the range between 500
and 600 nm (Vetter et al. 1971). Such carotenoids
are frequently found in bird feathers (Voitkevich
1966; Durrer 1986), and also in the oil droplets
of bird retina (see Leibmann and Granda 1975;
Goldsmith et al. 1984; Jane and Bowmaker 1988).
The type A feathers of red, yellow and green colour
probably contain this kind of carotenoid.
Feathers with spongious keratin containing irregularly spaced air-filled cavities will reflect all
wavelengths and hence appear white. These
feathers would comprise the type A white. The
presence of a small amount of melanin in these
feathers could provide the basis for the type A/B
white. The origin of the blue colour of many
feathers could be very similar. As in white feathers,
light is scattered by the outer keratin layers. But
in contrast to the situation in white feathers, deep
layers of melanin will absorb the farther penetrating long wavelength radiation. Thus the stray light
will become blue due to the Tyndall effect. Therefore, it is not surprising that in blue and violet
feathers an additional reflectance is present in the
UV, as the Tyndall effect becomes stronger at
shorter wavelengths. (For another model of colour
production in blue barbs, due to interference, see
Dyck 1971.)
Only rarely are green feathers coloured by pigment, in most cases the green colour originating
from the combination of a yellow pigment and the
structural blue described above. If the yellow pigment is of the carotenoid type a high UV reflectance would not be surprising, consistent with type A green. If the pigments contain melanin, UV
will be absorbed, probably resulting in green of
types A/B or B, depending on the amount and distribution of melanin.
794
Finally, coloration of feathers may be caused
by interference of light on structural layers oriented in parallel at a quarter wavelength's distance.
Interference colours may range from violet to red
depending on which wavelengths match the structural distances.
Single-banded interference colours did occur in
the samples as well as colours with side bands,
findings which are consistent with the results of
Durrer and Villiger (1966). In green feathers of
iridescent appearance feathers both with and without additional UV to violet side bands were found
and they were classified accordingly as types A or
B interference green.
Quite independent of which hue is being considered, the colour of a feather as seen by man
might be caused either by a comparatively small
reflectance band in our visible range [for red see
the scarlet ibis (Fig. 2); for green see the Indian
peafowl (Fig. 6); and for blue-violet see the Asian
fairy bluebird (Fig. 8)] or by a rather wide reflectance band with a depression in a narrow waveband. For example, the yellow feather of the budgerigar (Fig. 4) appears yellow to the human eye
as the complementary colour (blue) is absorbed.
Likewise, the budgerigar's blue feathers absorb the
orange part of the spectrum.
The different types of wavelength-dependent
reflectances imply that Munsell notations do not
give appropriate information about spectral properties of feathers. This is even more obvious when
the UV reflectance, which is not seen by man, is
taken in account, because in birds, especially those
with high UV sensitivity, it will considerably influence the perception of hues.
Tetrachromatic vision and plumage colours
as they may appear to birds
In most birds investigated so far at least four receptor types have been found (see Jane and Bowmaker
1988). This raises the question of how the colour
space in birds might be organized. For several reasons it seems unlikely that a chromaticity diagram
can be constructed as a quadrangle in a plane with
the four receptor-peak regions placed at its
corners. Such a configuration would imply that
white in the diagram's center could be reached by
an unlimited number of complementary wavelengths at opposite locations on the quadrangle's
sides. This would also imply that each of two pairs
of receptors not adjacent in the spectrum could
be stimulated appropriately to cause a perception
of white. Any location within the chromaticity diagram could be reached by stimulating three out
D. Burkhardt: UV-vision: a bird's eye view of feathers
/
P/
/ /'~.
L
B
\
R
V
\
G
BG
Juv .... \i B
uv
"'"
7.,.",
:
",
i
~
!
~176
""'-.. \
UV
B
Fig. 11. Schematic drawings of different types of chromaticity
diagram. Thick continuous lines: regions of primary colours;
broken lines: intermediate spectral colours; dotted lines: colours exclusively evoked by mixtures of two different regions
in the spectrum. R, red; Y, yellow; G, green; BG, blue-green;
B, blue; UV, ultraviolet; P, purple; UB, intermediate colour
between UV and blue; UG, intermediate colour resulting from
adding UV and green spectral lights; UR, intermediate colour
resulting from adding UV and red light. Upper left: chromaticity diagram of human colour vision. Any point within the
colour triangle may be reached by adding the three primaries,
as indicated by arrows. Upper and lower right: hypothetical
chromaticity quadrangle for tetrachromatic vision including
UV as a primary colour. As can be seen by comparison of
the diagrams, any point within the chromaticity diagram can
be reached by adding different triplets of primaries. Lower left:
hypothetical chromaticity tetrahedron. In contrast to the chromaticity quadrangle, not one dotted line is possible, but three
intermediate colours can be found which are not included in
the spectrum. Any point within the tetrahedron can be reached
by adding four primaries
of four receptors, that is four different combinations of three receptors (Fig. 11 right). Hence the
relation between the wavelength content of a stimulus and the perceived colour would be even more
ambiguous than in trichromatic vision. Therefore,
I favour a chromaticity diagram constructed in
three-dimensional space, similar to that proposed
for fish (Neumeyer 1988). The four receptors
would be represented by the edges of a tetrahedron. This implies the presence of three regions
of intermediate colours in the daylight spectrum
(yellow, blue-green - both visible to us -, and a
mixture of UV and blue-violet, visible to birds).
Furthermore, while in man's chromaticity diagram
there is only one intermediate colour which does
not occur in the daylight spectrum, namely purple,
in tetrachromatic vision there would be three intermediate colours which are not present in the day-
D. Burkhardt : UV-vision: a bird's eye view of feathers
light's s p e c t r u m , namely, m i x t u r e s o f red a n d blue
(purple), o f green a n d U V , a n d o f red a n d U V
(Fig. 11). I n spite o f the speculative n a t u r e o f this
hypothesis s o m e conclusions a b o u t the a p p e a r a n c e
o f colours can be d r a w n f r o m o u r m e a s u r e m e n t s .
Different feathers t h a t a p p e a r white to h u m a n s will
differ in c o l o u r for a bird with U V vision, depending on their U V reflectance. Thus, a b s o r p t i o n o f
U V (types A/B a n d B white) will cause the feather
to l o o k c o l o u r e d to the bird, since the U V p a r t
o f the daylight s p e c t r u m is filtered out. G r e y
feathers a b s o r b i n g U V will h a v e the s a m e hue, as
only the three receptors in the r a n g e visible to m a n
are stimulated.
O n the o t h e r h a n d , while white feathers reflecting the whole s p e c t r u m except the U V were found,
no b l a c k feathers a b s o r b i n g all wavelengths except
U V were seen. But it should n o t be ruled out t h a t
there m i g h t be shiny b l a c k feathers with a reflectance b a n d in the U V , due to interference. So far,
in the present samples o f d a r k feathers reflecting
in the s h o r t w a v e l e n g t h range, only U V reflectance
c o m b i n e d with reflectance in the ( h u m a n ) violet
a n d blue r a n g e has been found.
Red, orange, yellow, green a n d iridescent green
feathers each occur in two types, with a n d w i t h o u t
additional U V reflection. Therefore, c o m b i n a t i o n s
o f red a n d U V as well as o f green a n d U V occur.
These d o u b l e - b a n d reflectances were m e n t i o n e d
a b o v e as p r o b a b l y giving rise to hues which are
n o t present in the daylight spectrum. A c o m b i n a tion o f red a n d blue reflectance as the third type
o f i n t e r m e d i a t e colour was present in a p a r t o f
the I n d i a n p e a f o w l feather. Thus, all o f the three
s e c o n d a r y c o l o u r types not present in the daylight
s p e c t r u m actually do o c c u r in the p l u m a g e o f birds
(the red a n d blue c o m b i n a t i o n is also frequently
f o u n d in colours o f fruits). It seems extremely unlikely that such feathers should be highly u n s a t u r ated for birds. This m a k e s m e believe t h a t the chromaticity d i a g r a m is o r g a n i z e d as a t e t r a h e d r o n
rather t h a n as a q u a n d r a n g l e in a plane.
T o investigate the biological significance o f
bird colours including the U V a p p e a r s to be a challenge. W e s h o u l d realize t h a t different feathers t h a t
l o o k fairly similar to m a n m a y l o o k very different
to a bird a c c o r d i n g to w h e a t h e r they are o f the
A, A/B, or B type, which are m a r k e d by different
U V reflectances n o t visible for m a n . F o r example,
snow reflects U V ; a s n o w y owl w o u l d be b a d l y
m a t c h e d to s n o w unless its p l u m a g e belonged to
the A type o f white (with high U V reflectance),
which it actually does. U V p a t t e r n s are invisible
to m a m m a l p r e d a t o r s , while they could be conspicu o u s for U V sensitive raptors. Thus, the questions
795
o f inter- a n d intraspecific colour signals, o f c a m o u flage, a n d o f a p o s e m a t i c c o l o r a t i o n should be reinvestigated in this context (see B u r k h a r d t 1988).
F u r t h e r m o r e , s o m e birds m i g h t be sexually dim o r p h i c in U V patterns, similar to s o m e butterflies. A p p e a r a n t l y , this r e p o r t raises m a n y m o r e
questions t h a n it is able to answer.
Acknowledgements. The study was supported by the DFG with
funds from the SFB 4, E 6. The Vogelpark Walsrode (D-3030,
FRG) kindly supplied feathers and allowed birds to be photographed. The Carl Zeiss Company (D-7082, Oberkochen,
FRG) generously gave access to the CMS 230 diode array photometer. E. Finger assisted during measurements and evaluation of data, and B. Kramer improved the English. I am grateful
to Ingrid de la Motte for her invaluable help.
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