Breeding & Genetics
Thrush Chest aka the Spot Gene (in cockatiels)
This article is a detective story of sorts, about a quest to find the reason for some odd markings on cockatiels. A complete answer to the question hasn't been found and maybe it never will be, but there's more information about it now than there was before.
I became an involuntary cockatiel breeder in 2008, when Buster and Shodu produced a clutch of eggs and my only choice in the matter was whether or not to let them hatch. I did let them hatch of course, and it turned out that there were some surprises in those eggs. Like "how did I get lutino chicks from two grey parents?" It was relatively easy to find the answer to that question. A more enduring mystery was "why does the youngest chick (Squeebis) have spots on his chest and little stripes on his head?" I asked around on cockatiel boards and no one could tell me anything about it, so I started to think that I might have a new mutation on my hands and spent several years trying to figure out what was going on.
It turned out to not be anything so dramatic as that of course. These markings are actually fairly common, but so little attention has been paid to them that they simply weren't on the cockatiel community's radar screen.
As I started my quest I had to call this coloration something, so I came up with the name "spot gene" to describe the underlying cause, and I talked about it in so many different places that these markings are now fairly well known by this name. It turned out that there's an older name for them, but first let's talk about what these markings look like.
AppearanceOver time I realized that there are several characteristic markings besides the chest spots and head stripes. The typical markings include:
- A little yellow/white "blaze" on the forehead
- Yellow/white feathers on the face near the eye and lower beak
- More speckling under the wings than usual, and the speckling near the edge of the wing is often visible even when the wing is closed.
- Light edges on the lower abdomen feathers, creating a "scalloped" look
- Spots (or less often, small stripes) on the chest. The spots vary from one bird to another in size, shape and placement. In hens, the number of spots may increase over time, and the number of spots may change from one molt to another.
- In a small number of cases, small stripes on the side of the head behind the cheek spot area
- Like the pearl mutation, females keep the markings for life while males lose them at maturity. Adult males develop "ghost pearls" on the wings just as they do in the pearl mutation, even thought they did not have markings in this area as juveniles.
It seems that only the better-marked birds develop chest spots, and even fewer develop the head markings. Buster and Shodu have had several more clutches (with my permission this time), and I think that maybe all of their children have the spot gene. I can't tell what's happening with their lutino chicks, but all their cinnamon or normal grey chicks have the blaze, the light feathers near the beak, and the underwing speckling. But the percentage that has chest spots is fairly small (25% or less; I haven't kept statistics) and only a couple have had head stripes. Most of my chicks are not as well marked as the two in the pictures above.
Early in my search
for information, I came across this quote on page 228 of George A. Smith's
Encyclopedia of Cockatiels (1978):
This is another
dominant mutation with only a very slight significance. The affected
cockatiel has some of its chest and belly feathers with a reduced amount of
melanin, so they give a speckled chest to the bird. As the opaline mutation
does not pearl the feathers of this area, a more attractive bird is produced
by combining these two mutations.
sounded like it MIGHT be the same thing, but the description is ambiguous
and no pictures were provided, so I couldn't be sure. In spite of talking to
some very experienced breeders who were active in major cockatiel societies,
it wasn't until 2014 that I was finally able to locate someone who knew what
thrush chest was and could confirm that the spot gene markings are also
known as "thrushing" (ACS
Other references to thrush chest that predate my quest are few and far between.
The first edition of A Guide to Cockatiels and Their Mutations by Cross & Anderson (not the longer, more famous second edition that added Terry Martin as the first author) says on page 9 that hens "often have shaded grey or speckled chests" which appears to be a reference to the spot gene. It doesn't say anything about juvenile cocks having the markings too, or adult cocks developing ghost pearls. The term "thrush chest" is never mentioned. Smith's 1978 book is the only print source I know that uses that term.
The internet doesn't talk about thrush chest either (except in spot gene conversations), with one exception. CockatielsPlusParrots says that it is terminology related to the pearl mutation. This appears to be erroneous, since it is at odds with all the other information on the subject and George Smith clearly indicated his belief that it was a separate mutation. I can not locate any other source that uses the term thrush-chested in connection with the pearl mutation.
Mutavi's list of genetic symbols for cockatiels includes a symbol for spotting, which piqued my interest about why it was there. But alas, it is apparently unrelated to the spot gene. A newer chart on the same website uses this symbol for the pied mutation.
George Smith says the inheritance is dominant but I have no evidence to indicate whether he is right or wrong. I'm not aware of any test breeding that has been done. Since the expression of the spot gene is so variable, I wonder if it might be co-dominant to the normal gene, with double factor birds being marked more strongly than single factor. I don't have enough information about my own birds (or anyone else's) to make a guess about this.
With my own original pair, the hen (Shodu) has some small spots on the chest and her mate (Buster) has ghost pearls on the wings but has never produced a pearl daughter. So it's likely that both of them have the spot gene, but I don't know whether they are single factor or double factor. A few years later I started breeding a new pair consisting of a pearl hen (Ladybug) and a whiteface pied cock with ghost pearling on the wings (Pip). I had high hopes of getting some pearl chicks, but instead I got more spot gene chicks. Evidently the ghost pearls on Pip's wings did not come from the pearl gene, and I have no idea whether Ladybug has the spot gene or not.
Possible relationship to other mutations
I have wondered whether the spot gene is an allele of pearl (a different variation of the same gene, with a weaker and somewhat different effect). Both have the same basic action, creating patterned markings on the feathers by removing the melanin in some areas. But there are significant differences in the location and style of the markings, which casts doubt on this idea. Both the spot gene and pearl may cause ghost pearls on the wings of adult males (who otherwise lose their markings) while females keep the markings for life, which is another similarity. I also wonder whether pearl is ancestral coloring that has been restored by a back mutation (more on this in the Coloration Mechanics article). If so, it's possible that the spot gene is a vestigial remnant of this earlier coloring regardless of how it's related to pearl.
There have been cases where birds who were expected to produce pearl babies produced spot gene babies instead, or produced pearl babies at a far lower rate than expected. There have also been cases where a pair produced some pearl chicks and some spot gene chicks. Whether pearl and the spot gene are alleles or not, it's possible that the spot gene might disrupt the action of the pearl gene in some fashion. Or maybe the pearl mutation is accompanied by many modifier genes, and the pearl gene sometimes fails to express itself properly because some individuals have "broken" modifiers. This isn't surprising if pearl is restored ancestral coloring. There would have been no selection for or against the modifier genes during the time that the main pearl gene was switched off, so it's expected that they would break down in part of the population.
There's another complication however. The spot gene seems to be strongly correlated with the whiteface gene. I haven't kept statistics, but I've talked to quite a few people who have birds with these markings, and roughly 90% of the birds were either visual whiteface or known to be split to it. There have been some cases were a spotted bird's pedigree was known for several generations with no whiteface in it, so evidently these markings can occur in the absence of the whiteface gene. But most of the time they seem to travel together. These markings occur in combination with other mutations of course, but there isn't a strikingly consistent relationship with anything but whiteface.
The best explanation that I can think of for this apparent relationship is that the spot gene and whiteface are linked genes, meaning that they're both on the same chromosome, and they are so close together that they are rarely separated by a crossover. Maybe the "founder" bird who spontaneously developed the whiteface gene also happened to have the spot gene at a nearby location. But for this to be a satisfactory explanation, the descendants of all the other birds who had the spot gene before the existence of whiteface would have to stop displaying the trait for some reason.
If the spot gene is linked to whiteface then it can't be an allele of pearl, since the pearl and whiteface genes are definitely not on the same chromosome. But the spot gene could still be a pearl modifier, or it might be an unrelated gene whose function produces results that are superficially similar to pearl.
If whiteface and the spot gene are NOT linked then I'm at a loss to explain why they occur together so often. The whiteface gene controls psittacin (yellow/red) pigment and apparently has nothing to do with melanin pigment. The spot gene obviously affects melanin, presumably creating the patterned markings by switching melanin production on and off during feather growth (see the Coloration Mechanics article for more info on the process). These are completely different functions, and the presence of one of these genes shouldn't have any influence on the action of the other. There's no logical reason why the whiteface gene, especially a whiteface split, should help "bring out" the spot gene markings.
However it does look like some of our basic assumptions about genetics may not be as cut and dried as we think. There is a current trend in serious cockatiel breeders to produce normal greys with no splits for use in developing rare mutations. It's been reported that these birds have darker plumage and less intense psittacin coloring than the average captive-bred cockatiel, and that photos of wild cockatiels show coloring similar to these split-free birds. So it's possible that some mutation splits might contribute to a lightening of the melanin coloration and a general "yellowing up" of the bird, even though we normally assume that these splits have no effect at all because they are recessive. It has also been observed that a whiteface split seems to help counteract this yellowing-up effect, so it may have some influence. Perhaps a lot of our "recessive" mutations would be better described as weakly co-dominant.
Origin and history
In any case, the spot gene has evidently been around longer than either the whiteface mutation or the pearl mutation, so any connection between it and these mutations developed later. The pearl mutation first appeared in 1967 and whiteface in 1969. I have a 1962 edition of Experiences with my Cockatiels by Mrs. E.L. Moon that has pictures of spot-chested birds in it.
It's very questionable whether the thrush chest/spot gene trait should actually be called a mutation. These could very well be wildtype markings, perhaps a regional variation similar to an unrecognized subspecies, or an individual variation that's widespread in the general population. Post #27 in this Talk Cockatiels thread talks about wild-caught cockatiels with chest spots and stripes, some of them very pronounced. However the Australian breeder who had the birds did not provide pictures, so I consider this to be an unconfirmed report.
There are also reports of imported cockatiels from South Africa in the 1970s commonly having well-developed spot markings that sometimes circled the neck like a collar (Russo, ACS Facebook thread), and it's said that the markings are much less pronounced now. Maybe the South African breeders made more effort to produce nice markings. Or maybe the introduction of so many other mutations and splits has helped to diminish them somehow. I can't think of a logical reason for why this would be true, unless other mutation genes have more of an influence than we give them credit for. If one or more of these genes has a pleiotropic effect (meaning that it serves more than one function) or if it affects the production and distribution of something that's needed for the spot gene to function correctly, it could have an influence on the expression of the markings.
It's unfortunate that this trait has been neglected so long by the cockatiel community, because well-developed markings can be very striking and attractive. Apparently it's been sort of a "lost" mutation for a long time - it didn't go away but it became less dramatic through neglect, and most people stopped noticing it and forgot about it. But there are still some people who saw it at its best and remember it. It's not too late to reverse the trend and start breeding selectively to redevelop the trait. We would learn more about it in the process.
|Three views of the same bird. Photos courtesy of Sarah Maria Hoogwater||With this chick, the underwing spots spill down onto the side of the body, which is unusual. Photo courtesy of Marly Rivera.||A heavily marked bird. Photo courtesy of Vicky Hosman.|
Copyright 2015 Carolyn Tielfan all rights reserved