Often in discussion about breeding Self birds the question comes up about marked Selfs and why they unexpectedly appear in nests.
I don't know the answer for certain and can only speculate on the reason but read what I think and perhaps if you have your own ideas you can let me know and I will put them here for further discussion.
All things being equal when we pair for example A Self Red Brown to A Self Red Brown we should be able to confidently expect the offspring to arrive as perfect versions colour wise of their parents.
Occasionally we find a mis-marked youngster in a nest carrying white to some degree or another, the usual form this takes is a white belly patch or two small "horns" either side of the beak a chin spot under the beak,or the odd white flight.
My example above (bred this year 1999 from a line that has never shown this before) has all these faults.
If we study the recent history of the Bengalese we discover that before the 1950's the Self Bengalese was unknown to fanciers. We learn they were developed by Danish fanciers by the selection of lightly marked Varigated birds, through dedicated breeding and their undouted skills they produced the first unmarked Self Birds. These birds spread into Germany where they were improved and then into Holland and Belgium for further attention from the exhibition branch of the fancy, from there to the UK and USA.
Discovering these facts about their origin has led me to believe that there is no "genetic" factor that can be called Self, a Self bird even today being produced by the selection of birds that have a "tendancy" to produce Selfs, by a strong factor in their makeup that suppresses the appearance of varigation,in fact all Bengalese are genetically Variagated birds in my opinion.
We then occasionally pair birds were the factor for suppression takes a back seat and allows the odd varigated bird to put in an appearence.
What can we do with these birds in our breeding plans, not much, I have seen them used and produce Selfs but I would not advise this, the potential to produce mis-marked birds is always present, don't help it by using them in your breeding plans.