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The
check pattern in domestic pigeons
For DARWIN, the blue-bar rock pigeon was the starting point of the
domesticated domestic pigeon. Later colourings would have developed
from blue-bar pigeons in the course of domestication. This is also
the statement of the ornithologist BECHSTEIN, who kept pigeons
himself. He believes to have observed the appearance of the check
pattern, along with other colour changes, in semi-wild pigeons in
Thuringia (BECHSTEIN 1795). In contrast, CHARLES OTIS WHITMAN
(1842-1910) placed the check pattern at the beginning. For him, the
bar pattern of the rock pigeon resulted from a 'gradual progressive
modification' in the evolutionary process. He points out the
similarity of the check pattern in the wild pigeon species, which is
particularly evident in their juvenile plumage. The development from
check to bar would also be embryonic development. In the mating of
checkered domestic pigeons, descended from the rock pigeon, over the
generations, the checks would be reduced to 4, 3, 2, 1 and 0 bars,
while in his experiments over 8 years the reverse path, from bars to
checkered birds, had remained unsuccessful (WHITMAN 1916, p. 16ff.,
162). From today's genetic point of view, however, the pathway
observed by Whitman
in the experiment, from check to bar, is not unusual if the initial
population included a heterozygous-breed individual. A more recent
molecular genetic study speculates that the check pattern was
transferred from the Guinea pigeon to the domestic pigeon after
separation of the species (an introgression). According to one
estimate, this was only 429-857 years ago (VICKREY et al. 2018). It
is therefore interesting to follow the distribution and
documentation of checkered pigeons in the literature.
Fig. 1: Inheritance of
checker. Source: Critical Issues Part VII
Dissemination and documentation of the check pattern
In
today's city pigeons and when domestic pigeons mix with rock pigeon
populations, pigeons with the check pattern make up a large
proportion worldwide. Among Viennese city pigeons, HAAG-WACKERNAGEL/HEEB/LEISS
(2006) found a proportion of checks (31.7%) and dark-checks (24.8%)
compared to 37.3% that were barred. 4% were barless. Due to
epistatic effects (e.g. the spread factor covering pattern), 9.9%
could not be assessed. When observing feral domestic pigeons in
Kansas in 1984/85, JOHNSTON/JOHNSON (1989) found 37% barred pigeons,
compared to 22% checks and 41% dark checks. In Bangladesh wild
pigeon populations mixed with domestic pigeons, there were, among
other colors, 11% blue-checks and 75% blue-bars (KABIR 2016).
In
the early days of organized pigeon breeding and in the first
monographs on pigeons, checkered pigeons had no importance among
pigeon breeders. They were not highly regarded, with the exception
of some factor combinations with bronze or white in the check
outlines. BECHSTEIN (1795/1807) describes it as a variant of field
pigeons. In the first extensively illustrated German-language
domestic pigeon book by NEUMEISTER (1837), there is not a checkered
example among the 123 pigeons shown on the plates. However, the gene
for checks may have been present in the so-called white blaze with
whitish or bronze wing shields. In the text, the drawing contour of
the checks appears under the 'meliert' pigeons listed after the
field pigeons in the text, based on BECHSTEIN. Among these also
mentioned are larks and laced ones. The former were probably
preforms of lark-pigeons, the latter of the later scaled (laced)
lynx pigeons. BREHM (1857, p. 92) describes 'carp-scaled' and
'hammer-faced' pigeons among the field pigeons with which they were
associated but formed a minority. PETER PAILLOU had already
presented an early drawing of a Parisian Pouter with similar scales
in England in 1744. Similar to the Pigeon Maillés created from
pouters in BOITARD/CORBIÉ 1824 (p. 179ff.) as preforms of the later
Cauchois.
 
Fig. 2: Paris Pouter with
the check pattern by Peter Pailou 1744. Fig. 3: Color Pigeons at
Neumeister 1837
However, drawings of blue checks existed even earlier. If you go
back 429 years, the lower limit for potential introgression
mentioned above, then you are in the time of MARCUS Zum LAMM
(1544-1606). In Freiburg (Germany) he had pictures painted for his
Thesaurus Picturarum and put together a collection. Also mentioned
in his notes is a 'Visch Schüppichte or hammerschlegichte Daub'.
This was apparently a common name for checks in the region. One of
his pictures that has been preserved is a drawing of a blue-check
field pigeon. There must therefore have been checkered domestic
pigeons long before this time, as they were already common terms at
the time. If we go back 857 years, we are shortly before the time of
EMPEROR FREDERICK II (1194-1250), who wrote his Falcon Book between
1241-1248. It was completed by his son. In addition to turtle doves,
the numerous miniatures also contain two domestic pigeons, which can
be classified as checkered when compared to the blue-bars shown.
Even further back, the restored murals in Akhenaten's North Palace
in Amarna (around 1350 BC) in Egypt show, among other things, a blue
pigeon, which HAAG-WACKERNAGEL (1998, p. 46f.) classifies as
checkered. Overall, the evidence from the past gives the impression
that checks in domestic pigeons occurred mutatively in different
places at different times, as BECHSTEIN describes it.
 
Fig. 4: Blue check field
pigeon, Marcus Zum Lamm about 1600.
Source: Kinzelbach/ Hölzinger 2000. Fig. 5: Blue check pigeon in the
North Palace of Echnaton in Armarna, Egypt about 1350 BC. Source:
Haag Wackernagel 1998
 
Fig. 6: Checkered Dovecot Pigeon (Albin 1735). Fig. 7: Checkered
Dovecot Pigeon (Dixon 1851)
Good
conditions were found for checks, kept semi-wild, apparently at the
time of ALBIN, who drew a checkered pigeon in 1735 as a
representative of the 'Dove House' pigeons in England. A good 100
years later, DIXON describes her as a typical inmate of the English
'Dovecots' (DIXON 1851, p. 162ff.). According to DIXON, the
checkered variant had already successfully established itself under
railway overpasses in London and other parts of England. This
suggests advantages for the checks in the fight for survival as
urbanization increases.
The
increase in reputation and thus the spread in hobby breeding will be
linked to the spread of the Belgian racing pigeon after 1800. In
connection with this, from the second half of the 19th century
onwards, the breeding of racing homer-related fancy breeds such as
the Show Antwerp, Show Homer, Show Racer, the German Beauty Homer,
etc. On the 50 color plates in the magnificent work by FULTON (1876)
there are two color plates with checkered homing pigeons (in England
called 'Flying Antwerp' because of the imports via the port of
Antwerp form Belgium), and Show Antwerp. On the other panels there
is only one checkered one, namely a blue check shield owl. In
combination with white scales, the gene for checks can also be seen
in pictures of Oriental Owls, Hyazinth, Starlings and Ice Pigeons.
 
Fig. 8: Show Antwerp and Flying
Antwerp (Belgian Racing Homer). Fig. 9: Oriental Owls with the bar
and with the check pattern. Fig. 10: Hyazinth Pigeon and Starling
with the check pattern. Source: Fulton 1876
Hybrids
with Guinea pigeons
WHITMAN had already pointed out the similarity of the checkering of
the Guinea pigeon with the typical light triangular spot at the end
of the feather in the shield with the checkered of some, but not
all, domestic pigeons. A similar checkering can be found in many
wild pigeon species. In addition to the widespread turtle dove,
whose checks WHITMAN considers to be the archaic form of checks in
the pigeon family (p. 50), the special checkering of the Guinea
pigeon can also be found, among others, in Columba maculosa and C.
albipinnis (p. 163).
Hybrids of Guinea pigeons with domestic pigeons and reverse matings
were analyzed in blood group studies of pigeon species. So, in 1936
by IRWIN, COLE, GORDON as well as MILLER/BRYON 1953 and LABAR/IRWIN
1967. Among others, five different antigenic substances were
identified as putative inheritance units that were specific for the
Guinea pigeon (MILLER/BRYON 1953, p. 407). Hybrids with barred
domestic pigeons showed the check pattern and also the loyalty of
the racing pigeons to the location of the own dovecote (COLE CREEK
2019). Problems in maintaining and raising hybrids and early
mortality are reported (see also GRAY 1958). Some of the hybrids are
viable and capable of reproduction, even when mated back to domestic
pigeons.
  
Fig. 11 and 12: Guinea
pigeon and Hybrid-hen, lost by illness. Held and flown with the
Racing Homer team. Source: Cole Creek at Facebook. Fig. 13: Columba
maculosa, Lip Kee Yap, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>,
via Wikimedia Commons.
Diffusion of mutation and introduction of genes through hybrids
Similar behavior, the possibility of obtaining fertile hybrids, and
the visual similarity to the checks of the domestic pigeon led to
speculation that the pattern was transferred from the Guinea pigeon
to domestic pigeons. In view of the sparse references specifically
to the checkered pattern in domestic pigeons from the past, this is
not an unfounded hypothesis. Mathematical model calculations show
how quickly superior genes, whether created through mutations in a
population or added through crossing, can prevail when partners are
chosen at random. A dominant factor will reach a proportion of
around 50% after 400 years with a constant population and number of
offspring (per parent) of 5, an advantage of the carriers of the new
trait of 1/100 and an initial frequency of the new gene of 1/1000 (Mittmann
quoted from KÜHN 1961, p. 250). Random mating can be assumed in
feral domestic pigeons and, with regard to coloring, in racing
pigeons that are primarily selected for performance.
For other breeds, breeders' different selection
criteria play a role. In breeding groups in which new factors have
been introduced, the factor will combine with other existing ones
over the generations. Let's assume that in a population of pigeons,
Smoky is present alongside the wild type. Then after a short time
the proportion of pigeons with the smoky factor should not differ
between checkered and barred pigeons. Close genetic linkages that
could hinder mixing over centuries are unlikely. Regionally, there
will be differences in genetic makeup in subpopulations due to
spatial distances, other environmental conditions, factor
interactions that are not directly recognizable, threats from birds
of prey, etc. (SANTOS et al. 2015). Such differences have also been
found between feral domestic pigeon populations in the megacities of
the northeastern United States from Boston to Washington (CARLEN et
al. 2021). For example, in tests in which the proportions of certain
factor combinations are important, it will make a difference if, for
example, barred individuals are chosen from a different region or
from different breeds than checkered individuals.
In
the case of introgression, the main difference to mutation within
the species is that, along with the gene under consideration, the
hybrids are heterozygous for other hereditary factors. Some of them
were not present in the host population until then. Most of them are
probably not visible externally. They could be immunities, genes
that influence energy metabolism, etc. They could be
species-specific and, in the species that transmits the gene, have
emerged mutatively after establishing as a separate species. If
transmitted to domestic pigeons, many factors will disappear
quickly, but others will become established and possibly expand
significantly. The diffusion is likely to start from the point of
origin, similar to a mutation, and spread regionally over time.
However, under human care, distances do not play a role for newly
discovered mutations, as the Reduced example shows (SELL 2012,
2021). In subpopulations into which these factors have penetrated,
they will freely combine with each other and with those present, as
in the spread of mutations.
Measuring introgression
ABBA-BABA tests: In evolutionary
research, potential introgression is usually analyzed using
ABBA-BABA tests, in which four populations are compared with each
other (DURAND et al., 2011). There are two currently existing
populations P1 and P2. Then an evolutionary older population P3 and
a fourth outgroup population O. This is more distantly connected to
these three ingroup populations. Their genome serves as a reference;
their gene expression at the compared loci is designated A. The
alternative that P3 has is called the 'derived allele' and
symbolized by B. The null hypothesis for the empirical test is that
P1 and P2 diverged from a common ancestral population that had
separated from the ancestral population of P3 at an earlier time.
After P1 and P2 split off from the ancestors of P3, there was no
gene flow from any of the groups with P3. The alternative hypothesis
is that P3 exchanged genes with P1 or P2 after these two populations
separated (DURAND et al. 2011, p. 2240). If the null hypothesis is
true and the ancestral populations of P1, P2, and P3 were equally
likely to interbreed without selection differences, then the derived
alleles in P3 should match those in P1 and P2 with equal frequency
and the D-statistic of the ABBA-BABA test should be zero result.
Significant deviations from zero would require an explanation, which
could be an introgression from P3 to P1 or P2. Alternatively, from a
'ghost population' PG very similar to P3, which may no longer exist
(DURAND et al. 2011, 2040).
Repurposing the ABBA-BABA test to color classes or specific genes:
In the study by VICKREY et al. In 2011, the broader ABBA-BABA
methodology will be repurposed to address the specific question of
whether the domestic pigeon's check pattern was transferred from the
Guinea pigeon. In the experimental setup, P1 are barred domestic
pigeons, P2 are checkered domestic pigeons. P3 is the exclusively
checkered Guinea pigeon. The Wood Pigeon was chosen as the outgroup.
P1 and P2 are therefore not different species that mainly reproduce
among themselves, but rather different colors of the domestic
pigeon. In his studies in 1939, HARMS did not attribute their own
racial character to these (p. 11). The calculation requires the
identification of transferred (derived) genes or those that are
believed to be such. When mated randomly and maintained in symbiosis
over long periods of time, the expectation is that 'derived' and
putative 'derived' alleles will be distributed equally between
barred and checkered individuals.
Significance of the studies for color variations of domestic pigeons
The
checkered pigeon in the English Dovecots was given its own status by
BLYTH as C. affinis during DARWIN's lifetime, which DARWIN (1868)
denied with many arguments. He considered the barred version of the
rock pigeon to be the older one. For WHITMAN it was the other way
around (p. 49). When repurposing the ABBA-BABA tests to domestic
pigeons, checkered pigeons are treated as a separate population.
Coming from animal breeding, it's hard to imagine that barred and
checkered animals, which have lived in symbiosis for centuries, are
systematically different from one another. An exception is the genes
that determine the pattern. The regionally delimited Viennese city
pigeons, which form a reproductive community, may form a population,
but not individual colors from it. Unless there is a strong affinity
when choosing a partner for the same pattern. The study cites, among
other things, the study of feral domestic pigeons (ferals) by
JOHNSTON/JOHNSON 1989 to prove an affinity. This suggests, however,
shows that there is a greater preference of bar and check to
intermix through their choice of partner. In the fancy, breeding for
shows, the standard encourages a pairing of barred and checkered
animals. Heterozygous check individuals usually correspond more to
the standard expectations with open checks than homozygous ones.
This is also an incentive to pair both colors with each other.
On
the empirical side: For the gene region in which the patterns are
anchored, the D statistics show values close to one. In terms of
measurement, these gene areas of the checkered domestic pigeon
largely correspond to the gene areas of the Guinea pigeon and check
is the central 'derived allele' from the original question. One
difference is that no repeats of gene sections (copy number
variation) were found in the Guinea pigeon (VICKREY et al. 2011).
For the whole genome, D-statistics values close to zero were
determined. Positive at 0.021, which is considered an indication of
introgression from P3. It is not possible for outsiders to recognize
which phenotypes or characteristics are behind the suspected
“derived alleles” and how many there are in the sample. In the case
of rare genes, drift in the populations will cause problems in
clearly identifying “derived” alleles. Based on what is known so far
about genetic linkages and correlations and about the pigeon's
mating selection, deviations in D values from zero require
explanation. As with mutations, they could also be due to chance and
sample selection. Rare archaic genes, present or lost in varying
proportions across species due to genetic drift, could be confused
with derived alleles. Overall, a manageable number of individuals
were examined. Significance at low D-values can be achieved with a
moderate number of analysed individuals if several gene loci are
considered in each case. The mathematical sample size, which is
important for the formal significance statement, thus increases
multiplicatively.
Summary
The
proportion of checkered pigeons among domestic pigeons has increased
significantly in recent centuries. It is therefore interesting to
investigate possible causes and the question of whether the check
pattern got into domestic pigeons through mutation during
domestication or through hybridization with the Guinea pigeon. From
an animal breeding perspective, it is rather questionable whether
ABBA-BABA tests can be of any methodological help for this question.
Checkered and barred domestic pigeons do not form separate
populations. They are different colors of a reproductive community
in city pigeons and racing pigeons. Pigeons do not have such a
strong affinity when mating within identical colors that, according
to previous findings from crosses between breeds and studies of
genetic linkages, potentially acquired (derived) alleles remain
connected for centuries. Perhaps molecular genetic studies will soon
say more about this and/or something different. The question of
'derived' alleles is closely linked to the question of whether one
can imagine that mutations repeat themselves. This is assumed to be
excluded in DURAND's methodological presentation. If this is the
case, then the exclusive existence of the gene in the receiving and
releasing population, be it checkered or barred, can be a strong
indication, regardless of the value of the D statistic, viewed
alone. WHITMAN (p. 19) considered the check pattern to be an
ancestral feature of the phylum of pigeons, which was modified in
the barred rock pigeon by direct and gradual modifications. If the
programming for checks is preserved in the genome, the trait could
be activated by parallel selectively triggering mutations, which
could explain, for example, a surprisingly rapid parallel fixation
of traits in a parallel evolution of separate populations in
cichlids (Urban et al. 2020, p. 466). It is possible that parallels
can be found in other animal species.
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