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79
Not every white bird is an albino:
sense and nonsense about colour
aberrations in birds
Hein van Grouw
n the birding world, general confusion seems
to exist about colour mutations in wild birds
and the correct naming of these aberrations.
Almost all whitish aberrations are called ‘(partial)
albino’. However, most of these are not albino
and ‘partial albinism’ is – by definition – not
even possible. Some mutations are hard to distin-
guish in the field (and in museum collections)
because the colour of feathers with a pigment
reduction is easily bleached by sunlight and can
even become almost white. For the correct iden-
tification and naming of colour mutations, it is
necessary to know which changes have occurred
in the original pigmentation. But first of all, it is
necessary to understand which pigments deter-
mine the normal colours of feathers and how
these pigments are formed. All colour aberrations
described here have a genetic basis, ie, they are
caused by a mutation.
There are more possible colour mutations in
birds than those described below. However, the
ones dealt with here are those occurring most
frequently and are more or less clearly recog-
nizable, either in the field or in museum speci-
mens.
The aim of this paper is to present clear defini-
tions of the most common types of colour muta-
tions in birds and to give a basic insight into the
genetic or food-related mechanisms that cause or
influence these mutations. The paper is based on
the study of colour mutation genetics and the
study of many 1000s of bird skins and live birds
during the course of 15 years of working with
cage birds and as bird collection manager and
taxidermist of the Nationaal Natuurhistorisch
Museum Naturalis at Leiden, Zuid-Holland, the
Netherlands, as well as in other European natural
history museums (van Grouw 1997, 2000ab).
Pigmentation
The most important pigments that determine
plumage coloration in birds are melanines and
carotenoids. Carotenoids vary in colour from
pale yellow to scarlet red. They are taken in with
food and transformed into colour pigments by
enzymes. The deposition of the pigments takes
place directly at the start of feather growth.
Aberrations in this pigmentation are mostly
caused by a food problem and usually do not
have a genetic cause. Well-known examples are
flamingos Phoenicopteridae and Scarlet Ibis
Eudocimus ruber, which owe their respective
pink and red colours to the presence of red caro-
tenoids in their natural food. When these carote-
noids are in short supply, these birds will appear
white after the next moult. In the past, this hap-
pened frequently in captive individuals of these
species before caretakers understood this relation
between food and coloration.
In several European passerines, part of their
colours are caused by yellow and red carote-
noids, eg, Blue-headed Wagtail Motacilla flava,
Willow Warbler Phylloscopus trochilus, European
Blue Tit Cyanistes caeruleus, Great Tit Parus
major, Eurasian Golden Oriole Oriolus oriolus,
Common Chaffinch Fringilla coelebs, European
Greenfinch Chloris chloris, European Goldfinch
Carduelis carduelis, Eurasian Siskin C spinus and
Common Crossbill Loxia curvirostra. But also the
red in spotted woodpeckers Dendrocopos and
the yellow-green in green woodpeckers Picus are
caused by carotenoids. However, not every red-
dish coloration is a result of carotenoids; for
instance, the red underwing-coverts and flanks of
Redwing Turdus iliacus and the orange-brown
breast in European Robin Erithacus rubecula are
caused by a melanin (see below). Mutations
causing changes in carotenoid-based colour pig-
ments are rare but melanine mutations occur far
more often.
Melanins
Two types of melanin are present in birds: eume-
lanin and phaeomelanin. Depending on concen-
tration and distribution within the feather, eume-
lanin is responsible for black, grey and dark
brown feathers. In a high concentration, phaeo-
melanin is responsible for reddish-brown feath-
I
[Dutch Birding 28: 79-89, 2006]
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Not every white bird is an albino: sense and nonsense about colour aberrations in birds
99 Carrion Crow / Zwarte Kraai Corvus corone, Heeze, Noord-Brabant, Netherlands, 12 June 2005 (Rob G
Bouwman). Albino, juvenile wild bird. Complete absence of melanins in plumage, skin and eyes caused by
genetically determined absence of enzyme tyrosinase. Because Carrion Crow has no carotenoid-based colours,
whole plumage is colourless (white), as well as skin and eyes. Red of eyes is caused by blood shining through. In
nature, survival chances of such birds are very low because of bad eyesight of albinos.
100 Common Blackbird / Merel Turdus merula (Pieter van den Hooven). Albino, adult in captivity. Complete
absence of melanins in plumage, skin and eyes caused by genetically determined absence of enzyme tyrosinase.
Because Common Blackbird has no carotenoid-based colours, whole plumage is colourless (white), as well as skin
and eyes. Red eyes are caused by blood shining through.
101 European Goldfinch / Putter Carduelis carduelis (Pieter van den Hooven). Albino, adult in captivity. Complete
absence of melanins in plumage, skin and eyes caused by genetically determined absence of enzyme tyrosinase.
Carotenoid-based colours are still present.
102 European Goldfinch / Putter Carduelis carduelis (Pieter van den Hooven). 100% leucistic. Total absence of
melanins in all feathers but melanins are still present in skin and iris. Carotenoid-based colours are still present.
103 European Goldfinch / Putter Carduelis carduelis (Pieter van den Hooven). Diluted (isabel). Reduction in
eumelanin concentration, phaeomelanin is unaffected. Normally black plumage parts have turned grey. Because
this bird remained in captivity, there has been no further bleaching by weather and sunlight and visible colour is
result of mutation only.
104 European Goldfinch / Putter Carduelis carduelis (Pieter van den Hooven). Ino. Strong reduction of both mela-
nins: phaeomelanin (back) is almost absent, while there is hardly any oxidation of eumelanin. Normally black
plumage parts have turned pale brown. Carotenoid-based colours are still present. Because this bird remained in
captivity, there has been no further bleaching by weather and sunlight and visible colour is result of mutation only.
ers. In lower concentrations, the colour will
appear as yellow-brown to almost white. Both
melanins together can give combination colours
such as greyish-brown. In skin and eyes, only
eumelanin is present.
In some species, the colour is completely
caused by eumelanin, eg, Common Murre Uria
aalge, Razorbill Alca torda, Western Jackdaw
Corvus monedula, Rook C frugilegus and Carrion
Crow C corone. In most species, however, both
types of melanin are present. As far as currently
known, there are no wild bird species in which
only phaeomelanin occurs. In feathers that con-
tain both types of melanin, the eumelanin is
mainly found in the central part of the feather
while the phaeomelanin is mainly located in the
feather edges.
Most of the basic knowledge on biochemistry
and development of melanins already originates
from studies in the first half of the 20th century
(eg, Frank 1939, Mason 1953, Rawles 1953,
Lubnow 1963). The normal formation of melanins
starts after the first cell divisions of the fertilized
egg. Already in this early stage of embryonal
development, basic colour cells are formed
which soon migrate to the so-called pigmentation
centres. From these pigmentation centres, the
basic colour cells spread to, amongst others, the
feather follicles. The basic colour cells contain
the amino acid tyrosine. Controlled by the en-
zyme tyrosinase, the basic colour cells can pro-
duce melanins during feather development; the
basic colour cells are now called the colour cells.
Melanin formation involves a series of chemi-
cal reactions, starting with the oxidation of tyro-
sine, katalysed by tyrosinase. The melanin itself
is a polymere molecule which is also subject to
oxidation. The amount of oxidation varies: black
is the strongest oxidation form, brown a weaker
form. The colour cells deposit the pigments into
the feather cells through ‘extensions’.
The melanin formation process is determined
genetically. Any aberration in the process has a
potential influence on the colours of a bird.
Amutation in the formation of the enzyme tyro-
sinase may (partially) inhibit the formation of me-
lanin. Also a mutation causing a change in intra-
cellular conditions may have an effect on the
eventual outcome of melanin formation, eg,
causing a reduction in either phaeomelanin or
eumelanin. Changes may also occur in the distri-
bution of the basic colour cells which may result
in reduction or complete absence of coloration
in some feather tracts. And when a mutation
blocks the formation of extensions, the feathers
will also remain devoid of colours.
In summary, eumelanin is responsible for the
colours black and brown, depending on the
amount of oxidation. Differences in shade are
mainly caused by the concentration of pigments.
For example, the amount of oxidation of the
eumelanin in grey and black feathers is similar
but the concentration of eumelanin pigment gra-
nules is much lower in grey feathers.
Phaeomelanin is responsible for red-brown
pigments. Probably, phaeomelanin is also an oxi-
81
Not every white bird is an albino: sense and nonsense about colour aberrations in birds
99 100
101 102
103 104
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Not every white bird is an albino: sense and nonsense about colour aberrations in birds
105 106
107 108
109 110
83
dation product of eumelanin. The different
shades of this pigment as the human eyes see it,
from deep red-brown to yellowish-cream, also
depend on the concentration.
Albinism
Albinism is defined as a total lack of both mela-
nins in feathers, eyes and skin as a result of an
inherited absence of tyrosinase (Fox & Vevers
1960). It is probably one of the most frequently
mentioned colour mutations whereas it is, in
fact, one of the least frequently observed muta-
tions (especially in adult birds) in the field. Due
to a mutation, an albino completely lacks the
enzyme tyrosinase in its body. Tyrosinase is
necessary for the chemical process that produces
melanin pigments in vertebrates. Because of the
absence of tyrosinase, an albino cannot produce
melanin at all. The result usually is a completely
colourless bird (but see below the remark on
albinos with carotenoid pigments). Also, the eyes
and skin are colourless. The red or pinkish
‘colour’ of these parts is caused by the blood that
can be seen through the colourless tissue of eye
and skin. A mostly white bird which nevertheless
shows some form of melanin pigmentation is
never an albino, by definition. ‘Partial albinism’
does not exist and is a ‘contradictio in terminis’,
even if it is often used in the birding literature
(eg, Ogilvie 2001). It is simply impossible, just
like being ‘partially pregnant’.
Tyrosinase has no influence on the formation
of carotenoids. In an albino of a species with
carotenoids as additional colour, these pigments
remain present. Such a bird will remain com-
pletely or partially yellow (or red), dependent on
the natural location of the carotenoids in the
plumage. These birds do have red eyes and a
colourless skin. Contrary to popular belief, albi-
nos are therefore not necessarily all-white.
In all animal species, the mutation albino is
inherited through an autosomal recessive gene.
This gene is not rare and occurs in most popula-
tions. Albinos are born more frequently than one
would expect, given the rarity with which they
are observed. The reason that (adult) albinos are
observed so infrequently is related to their bad
eyesight. Due to the absence of pigments in the
eye, albinos are very light-sensitive and they
have difficulties in observing depth. As a result,
they are an easier prey for predators than normal
birds and also more readily fall victim to traffic
and other hazards. Most of them die soon after
the start of their independence, at fledging.
Leucism
Leucism is defined as a partial or total lack of
eumelanin and phaeomelanin in the feathers as a
result of inherited disorder of the deposition of
these pigments in the feathers. It is probably the
most frequently occurring inheritable colour
aberration in birds and it is most often – errone-
105 Eurasian Coot / Meerkoet Fulica atra, Capelle aan den IJssel, Zuid-Holland, Netherlands, 28 February 2004
(Chris van Rijswijk). 100% leucistic. Total absence of melanins in all feathers but melanins are still present in skin
and iris. In this almost completely white bird, virtually all feathers lack melanins.
106 Snow Bunting / Sneeuwgors Plectrophenax nivalis, Vlieland, Friesland, Netherlands, 15 November 2004
(Menno van Straaten). 100% leucistic. Total absence of melanins in all feathers but melanins are still present in skin
and iris.
107 Eurasian Oystercatcher / Scholekster Haematopus ostralegus (Pieter van den Hooven). 50% leucistic. Total
absence of melanins in some (random) feather tracts but melanins are still present in skin and iris. Note that deposi-
tion of melanins in affected feathers is blocked, not development of melanins itself. Therefore, unaffected feathers
have their normal colours.
108 Eurasian Oystercatcher / Scholekster Haematopus ostralegus, Texel, Noord-Holland, Netherlands, May 1974
(René Pop). Probably 100% leucistic. This white bird lived for many years on Texel, which makes an albino very
unlikely. Colour of eye and down layer have to be studied to identify it as a certain 100% leucistic bird and not a
bleached ino. In an ino, the down layer would be cream-coloured and the pupil ‘dark red’ (see main text). In a leu-
cistic bird, the down layer would be pure white and the eye would be normal (red iris with ‘black’ pupil). These
features can not be properly judged from this single photograph but considering the pure white plumage, a 100%
leucistic bird is the most likely identification.
109 Eurasian Coot / Meerkoet Fulica atra, Rotterdam-Ommoord, Zuid-Holland, Netherlands, 2 March 2003 (Chris
van Rijswijk). Diluted. Reduction of melanin concentration. Because Eurasian Coot only has eumelanin in its plum-
age, no distinction can be made in ‘pastel’ or ‘isabel’. Normally black plumage parts have turned grey, normally grey
parts are now paler grey.
110 Eurasian Jay / Gaai Garrulus glandarius (Pieter van den Hooven). Diluted (pastel). Strong reduction of concentra-
tion of both melanins, but they are still present. This results in very pale plumage but skin remains unaffected. Same
mutation as in House Sparrow Passer domesticus of plate 111. Because this bird remained in captivity, there has been
no further bleaching by weather and sunlight and visible colour is result of mutation only.
Not every white bird is an albino: sense and nonsense about colour aberrations in birds
84
111 House Sparrow / Huismus Passer domesticus, Camperduin, Noord-Holland, Netherlands, May 2000 (René
Pop). Diluted (pastel). Strong reduction of concentration of both melanins, but they are still present. This results in
very pale plumage but skin remains unaffected. There are many gradations of dilution but this is one of the more
extreme examples. Weather and sunlight have caused further bleaching. So, visible colour is result of mutation in
combination with bleaching by sunlight.
112 Common Starling / Spreeuw Sturnus vulgaris (Pieter van den Hooven). ‘Brown’. Incomplete oxidation of
eumelanin. Normally black plumage parts have turned dark brown. Phaeomelanin is unaffected. Incompletely oxi-
dised eumelanin is very light-sensitive and, in nature, ‘brown’ plumage will soon strongly bleach. However,
bleached ‘brown’ Common Starlings in adult-type plumage will not become as white as juveniles because of still
rather dark ‘starting point’ of adult-type feathers. Because this bird remained in captivity, there has been no
further bleaching by weather and sunlight and visible colour is result of mutation only.
113 European Greenfinch / Groenling Chloris chloris (Pieter van den Hooven). ‘Brown’. Incomplete oxidation of
eumelanin. Normally black plumage parts have turned dark brown. Phaeomelanin is unaffected. Dark (melanine)
ground colour of green parts is paler than normal. Incompletely oxidised eumelanin is very light-sensitive and, in
nature, ‘brown’ plumage will soon strongly bleach. Because carotenoids are unaffected, a bleached European
Greenfinch will gradually appear more yellow. Because this bird remained in captivity, there has been no further
bleaching by weather and sunlight and visible colour is result of mutation only.
Not every white bird is an albino: sense and nonsense about colour aberrations in birds
ously – called albinism or ‘partial albinism’. In
leucistic birds, the enzyme tyrosinase is normally
present and the production of melanin in the
basic colour cells and the transformation into
colour cells is normal. However, the deposition
of melanin in the feather cells does not occur
due to an inherited disturbance disorder of the
pigment transfer. As a result, more or less colour-
less (white) feathers occur at random anywhere
in the plumage.
Different forms of leucism are known and can
vary from only a few white feathers (<25%) to
totally white individuals (100%). White feathers
of leucistic birds are fully without melanin, with
no coloured patches (however, similar to what
has been described above under albinism, if
colours are present that are caused by carote-
noids, these remain visible). In certain forms,
only feathers become colourless while eyes, skin
and horny bare parts are normally coloured. In
other forms, these parts can become colourless
as well. Nevertheless, even birds with the latter
form of leucism still have dark eyes. The reason
for this is that only the pigments of the iris are
missing. The pigments in the back of the eyeball
remain present and therefore the eyes look dark.
So, leucistic birds always have coloured eyes
which distinguishes them from albinos. In all
forms of leucism, the eyesight is normal. Such
birds are not hampered by reduced eyesight, and
thus do not have reduced chances of survival.
Consequently, leucistic birds are observed rather
frequently.
Partly coloured feathers are very unusual in
leucism. Individual feathers that are partly
coloured usually indicate a bad condition of the
bird during feather growth and is not an inherit-
able character (ie, is not leucism). This is often
seen in, eg, Carrion Crows, especially those
eating junk food in cities.
Brown
Brown as aberration is defined as a qualitative
reduction of eumelanin. In this mutation, the
amount of pigment remains unchanged but the
appearance of the eumelanin is changed (cf Kopf
1986). As a result of an inherited incomplete oxi-
dation of eumelanin, black feathers will turn dark
brown. The phaeomelanin is unaffected.
In bird species in which both melanins occur
naturally, this mutation is less obvious than in
species which only have eumelanin. So, a
‘brown’ House Sparrow Passer domesticus (both
melanins present) appears less aberrant than a
‘brown’ Carrion Crow (only eumelanin). How-
ever, feathers with a qualitative reduction of
eumelanin are very sensitive to (sun)light and
bleach quickly and strongly. So, old plumage is
often almost white because the feathers are
bleached by (sun)light. For that reason, this
mutation is sometimes hard to distinguish in the
field. In the hand, such a bleached ‘brown’ bird
can be recognized by, eg, the colour of the down
layer under the contour feathers and the colour
of those parts of the remiges and rectrices cover-
ed by neighbouring feathers (often inner webs).
In these parts, sunlight cannot penetrate and
reduce the actual coloration.
The mutation ‘brown’ is widespread and can be
encountered regularly. The juvenile ‘biscuit-
coloured’ Common Starling Sturnus vulgaris
reported from Weerselo, Overijssel, Netherlands
(Dutch Birding 18: 244-245, 1996) is also an ex-
ample of this mutation. Many reports of so-called
‘leucistic’ birds actually refer to ‘brown’ birds. For
instance, the published photographs of pale Great
85
Not every white bird is an albino: sense and nonsense about colour aberrations in birds
111
112 11 3
86
Not every white bird is an albino: sense and nonsense about colour aberrations in birds
114 11 5
116 11 7
118 11 9
87
Grey Owls Strix nebulosa in Finland (Dutch
Birding 17: 19-20, 1995, 20: 296, 1998) do not
involve leucistic but bleached ‘brown’ birds.
In all species, the inheritance of the mutation
‘brown’ is recessively sex-linked. It is also the
only colour mutation that is sex-linked in all bird
species. This means that a brown mutant with
two normally coloured parents is always a fe-
male. In nature, ‘brown’ males are very rare
because they can only be born from a ‘brown’
mother and a normal father that is hetero-
zygotous for this mutation (and of course from
parents that are both ‘brown’). The chance for
this to occur in nature is normally very minute.
(Remember that in birds, males have two X chro-
mosomes and females have X and Y.)
Dilution
Dilution is defined as a quantitative reduction of
melanins. In this mutation, the amount of pig-
ment is reduced (cf Kopf 1986). The pigment
itself is not changed but due to a reduction in
pigment concentration, a ‘diluted’ colour is
observed compared with the original coloration.
Two forms of dilution can be distinguished.
Pastel
Pastel is defined as a quantitative reduction of
both eumelanin and phaeomelanin. Black feath-
ers will turn grey and reddish-brown feathers will
turn yellow-brown. The degree of dilution can
differ within a single species. But in general,
melanin reduction is c 50%. Such a mutant will
look like a bleached wild-coloured bird. Several
forms of pastel are known to occur within one
species, all based on different heritable traits. In
pastel birds, old plumage is often almost white
because the feathers are bleached by (sun)light
(as in the mutation ‘brown’). Bleached ‘brown’
and bleached pastel birds can usually be told
apart by examining the colour of the down layer.
Isabel
Isabel is defined as a quantitative reduction of
eumelanin only. Black feathers will turn grey.
The phaeomelanin is unaffected. In species with
only eumelanin in their plumage, such as crows
Corvidae, it is difficult to identify a mutant as an
isabel or pastel, because both mutations have the
same effect: a reduction of the concentration.
The difference can only be seen in the phaeo-
melanin and these species do not have that pig-
ment. Therefore, the general term dilution is used
Not every white bird is an albino: sense and nonsense about colour aberrations in birds
114 Northern Lapwing / Kievit Vanellus vanellus (Pieter van den Hooven). ‘Brown’. Incomplete oxidation of eu-
melanin. Normally black plumage parts have turned dark brown. Phaeomelanin is unaffected. Incompletely oxi-
dised eumelanin is very light-sensitive and, in nature, ‘brown’ plumage will soon strongly bleach to almost white.
In this bird, many old feathers have bleached strongly, a combined effect of the mutation and exposure to sunlight.
From a distance, it seems to have white and coloured feathers, and might be taken for a leucistic bird. However,
closer view reveals that the pale feathers are not white and the coloured feathers do not show their normal colora-
tion. In the hand, the down layer will be ‘brown’.
115 Common Gull / Stormmeeuw Larus canus canus, Maasvlakte, Zuid-Holland, Netherlands, May 1981 (René
Pop). ‘Brown’. Incomplete oxidation of eumelanin. Normally black plumage parts have turned dark brown.
Phaeomelanin would have been unaffected but Common Gull does not have phaeomelanin. In first-winter/first-
summer Common Gull, normally black colours are restricted to outer primaries and primary coverts and tail-band.
Grey of mantle has turned into pale creamy-brown. Remainder of brown feathering (especially wing-coverts) has
bleached to almost white. Feathers that are still brown are at rest covered by other feathers, and have been less in-
fluenced by sunlight.
116 Song Thrush / Zanglijster Turdus philomelos (Pieter van den Hooven). Ino. Strong reduction of both melanins:
phaeomelanin is almost absent, while there is hardly any oxidation of eumelanin. Normally black breast-spots
(eumelanin) have turned pale brown and are only faintly visible. Because this bird remained in captivity, there has
been no further bleaching by weather and sunlight and visible colour is result of mutation only. In nature, an ino
soon will bleach to almost white and may resemble an albino or 100% leucistic bird. Inspection in the hand of the
down layer (or other parts covered from sunlight) will reveal that it is the mutation ‘ino’.
117 House Sparrow / Huismus Passer domesticus (Niedersächsisches Landesmuseum Hannover, Germany) (Hein
van Grouw). Schizochroic (‘grey’) male. Phaeomelanin is completely absent, whilst eumelanin is unaffected. This
results in black or grey appearance.
118 House Sparrow / Huismus Passer domesticus (Pieter van den Hooven). Schizochroic (phaeo) male. Eumelanin
is completely absent. Phaeomelanin, which is largely restricted to the feather edges, is unaffected and even seems
brighter in absence of eumelanin.
119 House Sparrow / Huismus Passer domesticus (Pieter van den Hooven). Schizochroic (phaeo) female.
Eumelanin is completely absent. Phaeomelanin, which is largely restricted to the feather edges, is unaffected and
even seems brighter in absence of eumelanin.
88
in these instances, without making a further dis-
tinction.
Ino
Ino is defined as a strong qualitative reduction of
eumelanin and phaeomelanin. In this mutation,
phaeomelanin has (almost) disappeared and
there is hardly any oxidation of eumelanin. Black
feathers will turn very pale brown (almost white).
In fresh plumage, colour and pattern are vaguely
visible. Especially in species with a natural
amount of white in their plumage, it can be seen
that there is some pigmentation left in the
remaining non-white plumage. In an ino, the
plumage parts with normally the highest pigment
concentrations remain the most clearly visible
parts, for instance the black cap in Western
Jackdaw. Old plumage is almost completely
white in an ino because the feathers are bleach-
ed by (sun)light.
An ino has reddish eyes because pigments
have disappeared there as well. But the eyesight
of an ino is much better than that of an albino. It
can be stated with great certainty that any adult
‘white’ bird with red eyes in nature is an ino, not
an albino.
In most species, the inheritance of this muta-
tion is recessively sex-linked (see also the muta-
tion ‘brown’).
Schizochroism
Schizochroism is defined as the absence of only
one of both melanins. Two forms of schizo-
chroism can be distinguished.
Phaeo
Phaeo is defined as a complete reduction of
eumelanin (non-eumelanin schizochroism). In
this mutation, only reddish-brown phaeomelanin
is present in the feathers.
Grey
Grey is defined as a complete reduction of phaeo-
melanin (non-phaeomelanin schizochroism). In
this mutation, the plumage only contains the
black/grey and brown eumelanin.
These two mutations are rather rare. They can be
recognized especially in species with both
eumelanin and phaeomelanin in their plumage.
In many species, phaeomelanin is mainly located
in the feather edges. When eumelanin is absent
(phaeo), the darker feather edges result in a scaly
pattern. When phaeomelanin is absent (grey),
only black-grey and dark brown colours will be
visible, the red-brown to yellowish-cream
colours having disappeared.
In species with only eumelanin in their plum-
age, the recognition of a phaeo (non-eumelanin
schizochroism) can be difficult, because the
feathers of such a bird will be almost completely
white (ie, without colour) and the eyes are
coloured. Such a bird may resemble a 100% leu-
cistic bird but the feathers of the latter are usually
pure white.
Phaeo can be mistaken for the mutations
‘brown’ and ‘dilution’ in birds in old and bleached
plumage. As said earlier, feathers with reduced
eumelanin (both quantitatively and qualitatively)
are sensitive to light and can bleach to almost
white. All eumelanin seems to have disappeared,
and therefore such birds look like a phaeo.
However, they can be recognized by the colora-
tion of those parts of the feathers where light could
not penetrate (see also the mutation ‘brown’).
Again, examination of the feather parts where light
could not penetrate as well as the down layer can
often reveal the correct aberration.
Melanism
Melanism is defined as an increase of melanins.
Two forms can be distinguished.
Eumelanism
Eumelanism is defined as an increase of eumela-
nin. In this mutation, the total appearance of the
bird is blackish.
Phaeomelanism
Phaeomelanism is defined as an increase of
phaeomelanin. In this mutation, the total appear-
ance of the bird is reddish-brown.
Partial melanism sometimes occurs but this is not
caused by a mutation but by, eg, disease, mal-
nutrition or lack of exposure to sunlight. If these
causes are removed, normal feathers will appear
during the next moult. Melanism is one of the few
mutations in which there is no loss of pigments
but, on the contrary, an increase in pigment con-
centration. Birds with an increase in concentra-
tion of both melanins at the same time are not
known. Also, one melanin form does not replace
the other. For instance, phaeomelanin is not
transformed into or replaced by eumelanin. So, in
an eumelanistic bird, the amount of phaeo-
melanin remains normal but through the increase
of eumelanin concentration, the phaeomelanin
will not or hardly be visible. Eumelanism occurs
more frequently than phaeomelanism.
Not every white bird is an albino: sense and nonsense about colour aberrations in birds
89
Differently coloured
The term ‘differently coloured’ is used here to
indicate all other inheritable colour aberrations.
Two other aberrations are mentioned here be-
cause they occur quite frequently, although their
appearance can be very diverse, making recog-
nition difficult.
Grizzle
This is a leucism-like mutation. In contrast to leu-
cistic feathers, grizzled feathers are partly pig-
mentless.
Acromelanism
Acromelanism is defined as the deposition of
eumelanin and phaeomelanin in the feathers
depending on body temperature and environ-
mental temperature of the bird (Lubnow 1963).
This mutation is allelic (involving the same gene)
with albino. The coldest parts of the body, such
as the top of the head, have more pigmentation
than the warmer parts.
Concluding remarks
Giving a complete overview of possible inherit-
able colour aberrations in a limited amount of
space is hardly possible. The aberrant plumages
discussed above are grouped according to their
appearance so that they can be identified in the
field or in the hand. For instance, there are many
different types of leucism, all with a different
mechanism of inheritance and connected to differ-
ent genes, but all resulting in more or less colour-
less feathers. The same holds for the mutations
‘dilution’ and, to some extent, ‘ino’. It is, therefore,
impossible to give strict and exhaustive descrip-
tions and definitions for these groups of compara-
ble mutations: an exception can always be found.
Only the mutations ‘albino’ and ‘brown’ are
connected to the same genes in all species; and
the gene for ‘brown’ is always located on the sex
chromosome.
Acknowledgements
I thank Rob Bouwman, Pieter van den Hooven,
René Pop, Chris van Rijswijk and Menno van
Straaten for supplying photographs.
Samenvatting
NIET ELKE WITTE VOGEL IS EEN ALBINO: ZIN EN ONZIN OVER
KLEURAFWIJKINGEN BIJ VOGELS De belangrijkste pigmenten
die kleuren van vogelveren bepalen zijn carotenoïden
en melaninen. Carotenoïden zijn verantwoordelijk voor
gele tot scharlakenrode kleuren; ze worden via de voe-
ding opgenomen en met behulp van enzymen in kleur-
pigmenten omgezet. Afwijkingen in deze pigmentkleu-
ren zijn meestal een voedselprobleem en slechts zelden
genetisch bepaald. Melaninen zijn te verdelen in
eumelanine (zwartbruin pigment) en phaeomelanine
(roodbruin pigment). Melaninen ontstaan door een serie
chemische reacties, te beginnen met de oxidatie van het
aminozuur tyrosine, onder invloed van het enzym tyro-
sinase. De uiteindelijk gevormde melaninen zijn zelf
ook onderhevig aan oxidatie en dit oxidatieproces kan
zich in verschillende gradaties afspelen. Zwart is de
sterkste oxidatievorm, bruin is een zwakkere vorm.
Afwijkingen in door melaninen bepaalde kleuren zijn in
belangrijke mate genetisch bepaald. Er zijn allerlei ver-
schillende mutaties die op verschillende punten in het
vormings- en distributieproces van melaninen kunnen
ingrijpen. Enkele van de belangrijkste worden in dit arti-
kel besproken: albinisme, leucisme, bruin, dilutie (pas-
tel en izabel), ino, schizochroisme (phaeo en grijs) en
melanisme. Hierbij worden ook enkele hardnekkige
foutieve opvattingen over kleurafwijkingen uit de weg
geruimd, zoals de per definitie onmogelijke aanduiding
‘partieel albinisme’ (albinisme is een mutatie waardoor
het enzym tyrosinase ontbreekt, waardoor melaninen
totaal ontbreken). Vaak hebben vogels met een gedeel-
telijk wit verenkleed betrekking op leucisme. Afwij-
kingen die in literatuur als leucisme worden gemeld
hebben dikwijls betrekking op de mutatie ‘bruin’ of
dilutie (’verdunning’).
References
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Not every white bird is an albino: sense and nonsense about colour aberrations in birds
