Spectral reflectance and directional properties of structural coloration in bird plumage
School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK
* e-mail: d.osorio{at}sussex.ac.uk
Accepted 24 April 2002
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Summary |
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Key words: feather, colour, iridescence, interference colour, bird
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Introduction |
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Structural coloration in bird plumage is common and is especially important
in producing blues, greens and iridescence. Structural coloration can be
recognised in several ways; for instance, the colours of reflected and
transmitted light differ. Feather barbs work as interference reflectors
because they contain keratin (refractive index 1.56), melanin (refractive
index approximately 2.0) and air pockets (refractive index 1.0;
Land, 1972). Previously, it
was thought that laminar interference reflectors produce iridescent coloration
and that diffusing structural colours are due to incoherent scattering
(Fox, 1976
;
Finger, 1995
). However,
Rayleigh scattering does not account for the observed spectral tuning of
reflected light, which should give reflectance spectra dominated by short
wavelengths. To account for this discrepancy, Finger
(1995
) proposed that a
combination of scattering and selective absorption by pigments can account for
feather reflectance. There is no doubt that colours are caused by a
combination of pigmentation and structural coloration, e.g. the greens of many
parrots (Fox, 1976
). However,
Prum et al. (1998
,
1999
) suggested that
scattering is less important than previously thought. They found that diffuse
bluish colours in two parrots, a cotinga and an estrildid finch, were produced
by coherent reflection (i.e. wavelength-selective interference with refractive
index boundaries less than 1 wavelength apart) from a foam-like structure in
which spatial variation is the same in all directions.
Iridescence is `glittering or flashing with colours which change according
to the position from which they are viewed' (Oxford English Dictionary, second
edition, 1989). Colour in this sense means `hue' (for humans;
Wyszecki and Stiles, 1982)
and, for our purposes, iridescent coloration is characterised by variation in
the spectral location of reflectance maxima,
max, with
viewing geometry. Other features that often distinguishes structural
coloration from pigmentation are that saturated colours are reflected
directionally and that the brightest colours are not desaturated specular
highlights.
We set out to `unweave the rainbow' of structural plumage coloration with
some misgivings. This study does not directly add to knowledge of the physical
mechanisms, and surely their brilliance alone says much about the colours'
biological role. Iridescence attracts humans and presumably birds, where it is
often most prominent on adult males. However, carotenoid and melanin
pigmentation in the plumage appears to provide information about qualities
such as health and social rank (Olson and
Owens, 1998; Senar and
Camerino, 1998
; Badyaev and
Hill, 2000
; Badyaev et al.,
2001
), and it would be interesting to know whether the same
applies to structural, and especially to iridescent, colours. The iridescent
colours of starlings (Cuthill et al.,
1999
) and bluethroat (Luscinia svecica), blue-tit
(Parus caeruleus) and blue grosbeak (Guiraca caerulea)
`blues' are implicated in matechoice
(Andersson and Amundsen, 1997
;
Hunt et al., 1999
;
Keyser and Hill, 2000
), but
little is known about the possible costs of structural colours or how they
might reflect physical condition.
To understand how plumage colours vary and whether birds can detect this
variation, it is useful to measure reflectance spectra
(Vorobyev et al., 1998).
Spectra of pigment colours can be recorded with a fixed viewing geometry;
typically one that minimises the contribution of specularity (see Figs
1A,
2), but even so, a single
spectrum does not capture surface lustre. With iridescent plumage, no single
geometry can give a useful (i.e. non-black) reflectance spectrum for all
feathers (Cuthill et al.,
1999
), and a fixed geometry will not show how the colours change
according to view (see Fig. 3).
The problem is that theoretically very many measurements are needed to
describe structural coloration completely. The colours produced by diffraction
on the surface of a compact disc indicate the potential complexity of the
problem. Here, we aim to complement work on the structural bases of plumage
colours (Land, 1972
;
Finger, 1995
; Prum et al.,
1998
,
1999
;
Andersson, 1999
) by looking in
more detail at their spectral reflectance and visual qualities. We also
outline a simple way of measuring the structural coloration of feathers.
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Materials and methods |
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We also used a simple integrating sphere to give (approximately) uniform diffuse illumination. The mounting of the feather was the same as with the point source, but the feather was centred in a 260 mm diameter sphere, which was lined with a reflective coating of expanded polystyrene. This was lit by an 8 V quartzhalogen lamp 25 mm from the specimen, but separated from it by a baffle to block direct illumination, and on the axis of rotation of the specimen holder.
Reflectance measurements
A 1 mm diameter spot on the sample was focused by a quartz lens onto a 0.1
mm light-guide. This connected to a spectroradiometer (S2000, Ocean Optics)
which recorded from 300 to 800 nm. In the integrating sphere, low ultraviolet
intensity meant that measurements were unreliable below approximately 400 nm.
Reflectance was measured relative to a standard of freshly pressed
(medical-grade) barium sulphate, which was a nearperfect diffusing surface
(i.e. intensity was independent of surface orientation). When compared with
the diffusing standard, it should be noted that specular surfaces can give
reflectance values exceeding 1.0 (e.g. Fig.
4A).
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Viewing geometry
For a fixed observer viewing a fixed point on a surface illuminated by a
point source of fixed intensity, there are five independent variables that
might affect the surface colour (i.e. reflectance spectrum). The light source
can vary in azimuth and in elevation, and the object can rotate about three
axes. Dealing systematically with all these variables presents a formidable
problem, and we did not vary all five degrees of freedom
(Fig. 1A). Both the elevation
(E) and azimuth of the light source could be varied, but the specimen
could be rotated only about the horizontal axis perpendicular to the line of
sight (i.e. pitch). The angle of rotation is called surface orientation
(O; Fig. 1A). Also,
the feather was placed in two alignments in the holder, which we call vertical
and horizontal, according to the direction of its main axis when perpendicular
to the line of sight (Fig. 1B). In practice, we do not deal with effects of varying illumination azimuth since
this seemed to be redundant once the effective tilt of the reflective surface
had been taken into account (Fig.
2A; see below).
Photography
The five feathers described in some detail (see Figs
4,5,6,7,8)
were photographed (see Fig. 3)
with the same equipment used for recording spectra
(Fig. 1A) except that the
viewing lens was replaced with a digital camera (Canon D1; Nikkor 55 mm Macro
lens, aperture set to f-16). Pictures were modified digitally so that the
specimen filled a circular frame, even with oblique surface orientations.
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Samples
Feathers were from freshly dead or live birds. Those from live birds had
been obtained (under licence) for other purposes; dead birds had been killed
by cats, on roads, etc. Often, more than one colour was measured from a given
species.
We obtained reflectance spectra from structurally coloured feathers of 15 bird species: magnificent frigatebird Fregata magnificens; mallard Anas platyrynchos; common pheasant Phasianus colchicus; common peafowl Pavo cristata; rock dove Columba livia (feral pigeon); magnificent hummingbird Eugenes fulgens; black-chinned hummingbird Archilochus alexandri; common kingfisher Alcedo atthis; Indian roller Coracias benghalensis; bluetit Parus caeruleus; common magpie Pica pica; European jay Garrulus glandarius; rook, Corvus frugilegus; carrion crow Corvus corone; and common starling Sturnus vulgaris. Not all are mentioned specifically in the text.
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Results |
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Spectra were recorded from a fixed viewpoint, with variable elevation of the light source, E, surface orientation, O, and the alignment of the feather in the holder which was either horizontal or vertical (Fig. 1).
Summarising feather reflectance
Plumage reflectance spectra can be treated as having three components: (i)
caused by pigmentation; (ii) caused by spectrally unselective specular
reflection of the illumination, or `ordinary specularity'; (iii) caused by
structural coloration, especially interference reflection. Feathers often
combine all three components (Fox,
1976), but those illustrated (see Figs
3,4,5,6,7,8)
probably lacked appreciable pigment colour. Two had substantial ordinary
specularity (see Figs 6A,
7A) and all had structural
coloration.
Reflectance spectra of structural colours differ from pigment spectra,
especially the carotenoids and melanin pigments used by most birds
(Fox, 1976). Structural spectra
can be narrower (in waveband) (e.g. Fig.
4A) and have multiple spectral peaks (e.g.
Fig. 5A). Also, very few
plumage pigments preferentially reflect short-wavelength or green light
(Fox, 1976
). For convenience,
we recognise two main types of structural coloration: `directional' and
`diffuse'. Directional coloration is illustrated by hummingbird and pigeon
feathers (see Figs 4,
5). These feathers took the
colour of the feather pigment (often black) over a wide range of viewing
angles. Their reflectance spectra had a roughly sinusoidal form, one or more
peaks within the avian visible spectrum (300-700 nm). Rejection at spectral
minima was very good; for example, falling to approximately 1 % of peak
reflectance on the magnificent hummingbird crown (see
Fig. 4A). Spectra with a single
main reflectance peak were seen on hummingbirds and mallard speculum. Pigeon
nape feathers (see Fig. 5A) had
multiple reflectance peaks of roughly equal amplitude, as did magnificent
frigatebird, common pheasant and common starling feathers.
Whilst laminar structures produce directional coloration (see
Fig. 4 in
Land, 1972;
Land and Nilsson, 2001
),
diffuse coloration may be attributable to a more foam-like structure in the
feather barb (Prum et al.,
1998
,
1999
). This type of structure
should produce relatively broadly tuned spectra, which are not iridescent; the
blue of a European jay (see Figs
3,
6) is a good example. Other
feathers that we classify as having diffuse coloration come from the European
kingfisher and Indian roller (see Figs
3,
7,
8). They also have relatively
broad reflectance spectra but are, nonetheless, iridescent and have other
intriguing qualities.
Directionality
Both ordinary specularity and structural coloration can be directional. We
look first at how intensity varies with viewing geometry
(Fig. 2A) and then at spectral
variation (Fig. 2B,C).
Structurally coloured feathers (in our sample) diffuse light into an
approximately circular cone, which can be defined by its angular subtense,
, at 50 % of maximum intensity (Fig.
2A). With a perfectly diffusing Lambertian
surface, luminance is independent of orientation, i.e.
=180°. We
measured values of
ranging from approximately 40° for the
hummingbird crown (see Fig. 4)
to 180° for the jay (see Fig.
6).
In addition to the angle over which light is diffused, the direction in which it is reflected is important. The directionality of a planar mirror is given by simple optical geometry (Fig. 2A), but even without structural coloration a feather could be complicated. This is because a feather is not a continuous flat structure, but a lattice in which surfaces need not be parallel to the overall plane of the vane. In practice, however, when ordinary (spectrally unselective) specular reflection was visible (e.g. Figs 3C, 6, 7), its direction indicated that the reflective surface was in (or close to) the plane of the vane itself.
In contrast, the reflective `surface' producing structural coloration appeared, in some cases, to be tilted relative to the vane (Fig. 2A). This tilt was most obvious with directional coloration (see Fig. 4B) and was away from the main proximo-distal axis, not the lateral axis (as if, for a vertical feather, Fig. 1B, the structural reflectors resemble the louvres of a Venetian blind). This means that light is directed towards the feather's base or apex. Positive values of tilt, t, imply that the reflector is tilted so that light from above a near-horizontal feather is directed towards its base (Fig. 2A) and negative values imply reflection towards the apex.
Tilt of the structural reflector was 0° for feathers from the pigeon, magnificent frigatebird, common magpie and carrion crow. Non-zero values of t ranged from -20° for a pheasant neck feather to more than 40° for the magnificent hummingbird crown (see Fig. 4B). Across the peacock's eyespot, t varies so that, as the tail rotates, colours `switch on and off' separately, adding to the spectacle.
Variation in the reflectance maximum
The spectral location of maximum reflectance, max,
varied with viewing geometry in nearly all the structural colours we examined.
This is expected for laminar interference coloration, because the angle of
incidence affects the effective spacing of the layers (see
Fig. 7 in
Land, 1972
). However, feathers
differ from ideal interference mirrors, not least because they are not
optically flat (i.e.
>>0°;
Fig. 2A). We found that, given
a fixed viewpoint,
max was independent of orientation
O and was linearly dependent on the angular separation of the viewer
and illumination elevation, E. That is,
max=a-bE, where a is
max at E=0° (i.e. illumination and
line-of-sight co-axial). In our sample, the constant b was between 0
and 1.2 nm degree-1. It was interesting that the largest shifts in
max were from diffuse coloration, such as kingfisher and
Indian roller feathers (Figs 7,
8), rather than from
directionally coloured feathers.
The finding that max was independent of O was
unexpected because theoretical treatments (e.g.
Land, 1972
) do not deal with
diffuse interference reflectors. However, from optical geometry
(Fig. 2B,C), one can see that,
as the light source moves relative to the feather,
max(R), the reflectance peak on the axis of
reflection, is given by:
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We now illustrate some properties of structural plumage coloration with five selected examples (Fig. 3), the first two classified as directional and the other three as diffuse.
Magnificent hummingbird crown
The crown of the magnificent hummingbird
(Fig. 4) is a good example of
directional coloration, but we should start by noting that, when the
structural colour was not visible, the feather was almost perfectly black,
with a pigment reflectance of 0.025. Also, the feather was `matte', lacking a
spectrally unselective specular reflection
(Fig. 3A). With a low light
elevation, E=5°, and the feather turned to maximise this specular
reflection, the spectrally unselective reflectance added approximately 0.01.
By comparison, an ordinary `black' pigmented feather from a male European
blackbird (Turdus merula) produced a specular reflection of 0.04 (in
addition to a reflectance of 0.04 for the pigment colour).
This black `background' gave a strong visual contrast for the structural
colour, which was relatively directional; light from a point source was
diffused over a cone 40° across (Fig.
2A,
4B). Also, the feather was
black when illumination was (nearly) co-axial with the line of sight
(Fig. 3A). For a normal glossy
pigmented surface, this viewing geometry should give a relatively bright
colour. That this did not apply to the hummingbird feather was consistent with
the reflective structure being tilted relative to the plane of the vane. This
tilt, t, was approximately 42°, which meant that light from above
a near-horizontal crown feather would be reflected towards its base (i.e.
forward over the bird's head). Reflectance spectra had a single peak in the
visible range, max=440 nm, at E=5°, with a
half-width of approximately 70 nm. There were secondary maxima outside the
visible range, one probably at twice the wavelength of the primary peak. The
reflectance minimum was 0.025, indicating virtually perfect rejection. As the
light source moved,
max varied according to the formula:
max=442-0.40E
(Fig. 4C).
Under diffuse illumination, in the integrating sphere, the hummingbird
feather had low reflectance (maximum approximately 0.1) when perpendicular to
the line of sight and at all orientations when aligned horizontally (Figs
1B,
4D). By comparison, the
vertically aligned feather was brightly coloured when viewed obliquely. Also,
max was dependent upon orientation. Thus, even under
diffuse illumination, viewing geometry affects the colourfulness of this
feather; in nature, the `brightest' colours would appear when the bird was
level with and facing a viewer. This directionality under diffuse light is
intriguing and reminiscent of, but less marked than, that produced by the
butterfly Ancyluris meliboeus
(Vukusic et al., 2002
), in
which both diffraction and multilayer reflection are involved.
Feral pigeon nape
As with the magnificent hummingbird, the coloration of pigeon neck feathers
(Fig. 5) is directional and
probably produced by a laminar structure. When structural coloration was not
visible, reflectance was 0.05 and spectrally flat
(Fig. 5A), indicating that the
pigeon feather's pigmentation was paler than the hummingbird's. Spectrally
non-selective specular reflection was low, but difficult to measure.
Although directional, the pigeon's feather diffused light over a larger
angle (=70°) than did the hummingbird crown (Figs
2A,
5C). The reflective surface was
not tilted relative to the feather surface (t=0°), unlike the
hummingbird feather. A notable characteristic was that reflectance spectra had
multiple maxima of approximately equal amplitude. These peaks were separated
by a fixed frequency, forming a harmonic series
(Fig. 5B). For instance, with
roughly coaxial (E=5°) illumination and viewing angles, maxima
were at 310, 365, 441, 559 and 781 nm. The separation of these peaks was close
to a wave-number of 1/2000 nm-1 (i.e. 1.5x1014
Hz). As the light source moved,
max varied
(Fig. 5D), with the `violet'
peak shifting according to the formula:
max=438-0.48E.
Under diffuse illumination (Fig.
5E,F), unlike the hummingbird, the pigeon's reflectance spectra
were alike for vertical and horizontal feather alignments and were symmetrical
about the plane perpendicular to the line of sight. This symmetry is
consistent with the structural reflector being parallel with the feather
surface. max varied markedly with orientation (O),
shifting by approximately -2 nm degree-1 as the surface rotated
away from the plane perpendicular to the line of sight (Figs
1A,
5F). Given that, under point
illumination,
max was independent of orientation, this
effect might be seem surprising. The reason is that the feather is a
directional reflector so that, as orientation varies, so does the effective
elevation of the light source and, hence,
max
(Fig. 2B). Consequently, a
static viewer sees the pigeon's iridescent coloration change during displays,
at least when the bird is under diffuse lighting.
European jay wing covert
European jay wing covert feathers (Fig.
6) are striped, with white shading through bright blue bands into
black (Fig. 3C). This
structural coloration diffused reflected light more-or-less like pigmentation
(Fig. 6A). Also, the feather
had a spectrally flat specular highlight characteristic of a glossy surface,
which increased reflectance by approximately 0.25
(Fig. 3C). Again, as with
pigmentation, max was unaffected by viewing geometry; the
reflectance maximum was approximately 260 nm wide at 50 % of maximum intensity
(this value varied with position on the band). Under diffuse lighting
(Fig. 6B), reflectance spectra
resembled those seen under a point source.
The jay's blue coloration resembled the non-directional colours described
by Prum et al. (1998,
1999
). We now look at two
further examples of diffusing coloration, but these are more complex, being
iridescent and having other directional properties.
Common kingfisher crown
With a fixed light position, the kingfisher's crown feather resembled the
jay's feathers in having a diffuse, spectrally tuned reflectance and a
spectrally unselective specularity (Fig.
7A,B). Also, the reflectance spectrum was broad, but in this case
one or two comparatively narrow peaks rose above the plateau. However, the
location of the spectral peak was not fixed, varying according to the formula:
max=535-1.22E (Figs
3D,
7C).
In addition to the variation in max, the form of the
kingfisher feather reflectance spectrum altered with viewing geometry in that,
with illumination near the line of sight (E=5°), reflectance
varied more strongly across the spectrum than with illumination perpendicular
to the line of sight (E=90°;
Fig. 7B). As a result, the
kingfisher's colour tends to be most saturated when the bird is lit from
behind the viewer. There was no effect of feather alignment
(Fig. 1B) on reflectance
spectra, suggesting that the reflecting structure is in the plane of the
feather vane (i.e. t=0°).
Under diffuse lighting (Fig.
7D), the reflectance spectra were broader than with a point source
and max was insensitive to orientation and to feather
alignment. This is perhaps to be expected for diffuse coloration and can be
contrasted with the pigeon's nape, where
max varied with
orientation (Fig. 5E,F).
Indian roller tail
The feathers of the Indian roller's tail are dark blue with a greenish-blue
patch. The two colours share similar optical properties, and we describe the
greenish-blue (Fig. 3E). As
with the kingfisher crown, the reflectance spectrum had a broad plateau with
one or two comparatively narrow peaks (Fig.
8A,B). The location of the peak reflectance was given by the
formula: max=555-1.22E
(Fig. 8C). The directional
properties of Indian roller tail coloration superficially resembled those of
the kingfisher crown, but the way in which colour varied with viewing geometry
was different. For example, there was little spectrally unselective specular
highlight. Instead, both intensity and the ratio of maximum to minimum
reflectance (and hence `saturation') varied with viewing angle such that,
given a fixed light source, both brightness and saturation increased with
surface orientation (O; Figs
1A,
3E,
8B). Also, given a fixed
orientation, reflectance fell with increasing angular separation of the viewer
and the light source (E; Figs
3E,
8A).
These directional properties mean that the Indian roller's tail looks brightest and the colour most saturated when it is viewed obliquely and lit from behind the observer (Fig. 3E). As it turns towards the observer, the feather darkens and becomes greyer, which gives the impression of translucence. These directional properties cannot adequately be summarised by the scheme outlined in Fig. 2A but, as with the kingfisher, there was no effect of feather alignment (Fig. 1B) on reflectance spectra, again suggesting that the reflecting structure was parallel to the plane of the feather vane (i.e. t=0°).
Under diffuse lighting (Fig.
8D), the Indian roller tail resembled the kingfisher crown in that
spectra were broader than under the point source and max
was insensitive to orientation and to feather alignment
(Fig. 1). Both the maximum
reflectance and variation in reflectance across the spectrum increased with
surface orientation, which was consistent with observations under a point
source.
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Discussion |
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Feathers as interference reflectors
Laminar interference reflectors can give a range of spectral reflectance
curves depending upon the relative thickness, refractive index difference and
number of layers (see Fig. 4 in
Land, 1972). Land's models can
be used to generate spectra that resemble those we recorded from directionally
coloured feathers (Figs 4,
5). Interference reflectors
with multiple (more than five) equally spaced layers can produce both single-
and multiple-peaked spectra (Figs
4A,
5A,B; see
Fig. 4c in
Land, 1972
;
Land and Nilsson, 2001
).
Variation in the spacing of layers whitens the reflectance spectrum, which
means that spectra with good rejection at reflectance minima and hence
saturated colours probably require equally spaced layers. By
comparison with multilayered reflectors, a reflector with relatively few
layers should produce wider peaks with lower reflectance and, hence,
relatively dull colours (Land,
1972
). The iridescent glosses on corvid feathers including
European magpie, rook and carrion crow plumage had spectra of this
kind.
Of the examples of diffuse coloration illustrated, the jay wing covert
(Fig. 6) most resembles the
type described by Prum et al.
(1998,
1999
) in which the spectral
reflectance is independent of viewing geometry. The kingfisher and Indian
roller's coloration (Figs 7,
8) was diffuse but also
iridescent. These feathers might combine features of laminar and foam-like
structures.
How colours vary with viewing geometry
Iridescence and other directional properties are distinctive qualities of
structural coloration. Some features of plumage colours that affect their
visual appearance are as follows.
(i) Pigment reflectance was often very low, i.e. black. For example, with the feather perpendicular to the line of sight (O=0°) and a low light elevation, E=5°, the reflectance of the hummingbird crown feather at 500 nm was 0.025, whereas for a typical black bird, the male European blackbird (Turdus merula), reflectance was 0.04.
(ii) Coloration sometimes, but not always, lacked an ordinary specular highlight, i.e. the feather surfaces were matte. A matte black background means that the structural colour produces a high-contrast signal when the feather `catches the light' (Fig. 3A).
(iii) Iridescence is expected for interference colours because their
spectral tuning is dependent on the effective spacing of the structural
layers, and this varies with the angle of incidence
(Land, 1972). However, models
of flat mirrors do not predict how
max varies across a
diffuse beam from a surface such as a feather. We found that, under a point
light source,
max was independent of viewing angle
O and linearly related to the angle between the light source and the
line-of-sight E (Figs
1A,
2B,
4C,
5D,
7C,
8C). These relationships can be
understood if it is assumed that spectral tuning in a diffuse beam depends
upon the mean of the angles of incident and (diffusely) reflected light paths
through the feather barb (Fig.
2C).
The fact that when a surface is lit by a point source (e.g. the sun)
max is independent of viewing angle might seem surprising
because feathers can change in colour (hue) as a bird moves. The reason is
that although
max is independent of surface orientation
under a point source, for directional coloration under diffuse light, the
effective location of the light source (e.g. the part of the sky best
reflected by the feather) varies with orientation (Figs
4D,
5E,F). With diffuse coloration,
max is indeed fixed unless the viewer moves relative to the
light source (Figs 7,
8).
Another reason that the effects of viewing geometry on
max can lead to very different types of (visually
perceived) colour change is that the way colour varies depends upon the
spectral reflectance function. Single-peaked spectra
(Fig. 4) resemble spectral
lights, and colour changes are `along' the spectrum. However, multiply peaked
spectra (Fig. 5) can give
`non-spectral' colours such as purples or achromatic colours
(Finger and Burkhardt, 1994
).
This is why, for human viewers, the colours of a pigeon nape shift from green
to purple through grey (Fig.
3B). Birds have four spectral types of (single) cone
photoreceptors (Hart, 2001
),
and the `non-spectral' colours and the colour changes that they might see when
viewing multiply peaked plumage spectra may well not appear elsewhere in
nature.
(iv) The reflective structure producing coloration may be tilted relative
to the plane of the feather blade (Fig.
2A). This means that no single viewing geometry can be used to
maximise spectral signals from iridescent feathers
(Cuthill et al., 1999). Where
present, this tilt was away from the feather's main (proximo-distal) axis
(Fig. 1B). The tilt affects
when the structural colour is visible and, presumably, its use in displays. By
comparison, ordinary (spectrally unselective) specularity always appeared to
originate from a surface that was roughly coplanar with the feather blade
(Figs 6A,
7A).
The directional properties of the diffuse coloration of kingfisher and Indian roller feathers are less easy to understand than those of directional coloration, but they too affect their visual appearance. For example, the Indian roller tail look most brilliant (brightest and most saturated) when viewed obliquely and lit from behind the observer (Fig. 8).
Iridescent colours as signals
There is evidence that carotenoid-based pigmentation, giving red, orange
and yellow colours (sometimes with a secondary ultraviolet peak), influences
mate choice by birds. Also, the ability to obtain or use carotenoids may be
correlated with other less visually obvious measures of a bird's health or
`quality' (Olson and Owens,
1998; Badyaev et al.,
2001
). Are iridescent colours simply a cheap way of attracting
attention, or might they also convey information? The basic materials, keratin
and melanin, are common to all plumage, but it is possible that the formation
of a regular composite structure, which is necessary for a well-tuned and
bright reflectance spectrum, is energetically demanding. For example, the
multiple harmonically spaced spectral peaks seen on the pigeon's nape feather
(Fig. 5) are characteristic of
a structure formed of many layers (see Fig.
4 in Land, 1972
).
The modulation of reflectance across the spectrum is very sensitive to any
variation in the separation of these layers; unequal layering would produce
greyish rather than spectrally tuned iridescence. Also, the directionality may
depend on local and long-range order in the structure of the feather.
Alternatively, it might be that iridescence is a by-product of feather growth, perhaps produced by a robust process resembling crystallisation. Even so, colour could be informative if iridescent coloration is sensitive to damage. Answering these questions requires electron microscopy of feather barb structure and comparisons of iridescent coloration within a single species.
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