I've deliberately split up inbreeding and mating related. These two are strongly linked: mating related results in inbred offspring. Yet it is important to make the distinction and considering both when pairing up.
The extremes of both aspects are:
Both aspects are not absolute but a continuum from 0% to 100%. This is very important: an individual that may be inbred by some standard might be non inbred by another. It all depends where you draw the line in the continuum. But for simplicity I'll pretend both aspects are either black or white.
Here is a table where I combine the relationship between the parents and the inbreeding in the parents. I also mention the consequences on the genotype (genes) and phenotype (traits).
Table 1: Consequences of mating (un)related and inbred or non inbred parents.
| Pairing related birds | Pairing unrelated birds | Using inbred birds | Using non inbred birds |
|---|---|---|---|
| Double up on genes | Gene pairs are different | Predictable gene flow to the offspring | Recombines genes |
| Double up on good traits Double up on bad traits Creates prepotency: "pure" individuals that breed true |
Dilutes traits The only way to bring in new genes |
Uniform offspring (traits) Every trait gets passed, though it can be hidden |
Recombines traits A lot of variation in the offspring |
A good selection of the breeding team is far more important than any breeding system. But a well thought through breeding system can speed up the progress of improving your stud.
There are no guarantees. These are just theoretical thoughts based on statistical calculations. The only proof is in the eating the pudding. But nevertheless these can be used as guidelines.
The extremes of both aspects can be combined into four quadrants. Since there are two parents to each pair you would actually need a three dimensional representation (inbreeding father, inbreeding mother, relationship parents), but for simplisity I've combined the inbreeding of both parents into one dimension resulting in a nice two dimensional picture.
Inbred parents
^
Q2 | Q1
Unrelated ----+----> Related parents
Q3 | Q4
|
Non Inbred parents
Picture 1 : The combinations of inbred (I) / non inbred (N) parents that are related (R) or unrelated (U).
All quadrants can be useful but some, like Q2 and Q4, are more useful then others. You have to jump through different quadrants at the right time. Some quadrants are good at fortifying existing traits others at introducing new traits.
I'll now go over each of these quadrants individually.
This is obvious the most inbred you can go. Extracting the consequences from Table 1:
So basically this is creating more of the same. This can be necessary when you for instance have one exceptionally good inbred cock. Then you probably want to mate this cock to a related birds in order to create more exceptional birds.
Having both parents inbred and related is not a good idea. If you have multiple related and inbred birds you probably want to visit the opposite quadrant Q2.
If that exceptionally good inbred cock is an outcross in the first year you mate it unrelated (because you don't have any close related birds). The following year it is paired back to its daughters. This is in effect a Q1/Q4 combination.
I've personally done this with great success. To be fair, I've also done this with outcrosses that didn't produce anything useful. In this case the only thing you can do is get rid of all the inbred offspring.
It is a calculated risk and if it works out you're left with a line of excellent birds. The problem is that you are left with all inbred and closely related birds, so you'll need to go to an opposite quadrant next.
This is really combining two inbred lines. Extracting the consequences from Table 1:
The offspring will be fairly uniform. Every good traits gets passed, but it could be hidden in the offspring. So the quality of this offspring could be disappointing. However since every trait gets passed they may still be very useful in the breeding cage when mated back to on of the two parent lines (see Q1).
The best way to introduce a new trait in your stud is to get an outcross that is inbred for that trait. You will want to mate it to birds of your own that breed trough for their traits (i.e. are inbred). The results may in the first year be disappointing. See Q1 on how to progress from here.
This is a way of trying to strengthening the characteristics of an non inbred individual. Extracting the consequences from Table 1:
The creation of a lot of variation in the offspring with differently combined traits. As a result you'll need to cull heavily in the offspring. But you could also get that one super bird that combines all good traits. Such a bird would be a great start for a new line.
You are really playing the numbers game: with enough offspring you'll get some very useful birds next to a lot of mediocre ones.
Given the high variation it might be useful to use one inbred parent. Which is in effect a Q1/Q4 combination (see the example in Q1).
Such a pairing can be used for concentrating good traits and in that way step up the quality of your birds. Certainly if you don't have inbred birds yet, this can be a first step in creating a line.
As said the variation between the offspring can be big so you may not get birds with certain faults. Given that that offspring is inbred you'll need to be very careful with those faults as they will probably propagate to the offspring.
This is the typical outbreeding scenario. Extracting the consequences from Table 1:
The variation in the offspring is much lower than in Q4. This has to do with the fact that traits are diluted.
Culling will make the line only progress a little. You really need some inbreeding to aid and reinforce the selection. For instance if you've got a few split blue birds in your light green stud. If you do inbreeding the blue gene will pop up sooner then when you keep inbreeding to a minimum. This helps you in selecting for or against the blue gene. The same holds for a good (or bad) trait.
For inbred birds there are two quadrants that are applicable: Q1 and Q2. If the inbred bird is a super bird you might consider mating it related to strenghten its features in the stud (Q1). Preferably it is mated to a mate that is not inbred itself so it can bring in some new features. A pure Q1 mating: Mating two inbred birds related is not recommended.
Certainly for very inbred birds mating it to an unrelated bird (Q2) is the way to introduce new features into the line without loosing to much of the good traits of the line itself.
Non inbred birds are preferably mated related (Q4). Mating non inbred birds unrelated will make it difficult to surface the traits and to select on them. Together with a pure Q1 mating a pure Q4 mating is not recommended (though I would certainly prefer a pure Q4 mating over a pure Q1 mating).
Mating unrelated birds to produce non inbred birds is best done with inbred birds. This way they will have both of the good genes of both parents, either visually or hidden, and will enable you to use this non inbred offspring further in the original lines.
You could also mate unrelated birds where one parent is inbred and one parent is not inbred. This may be less ideal, but if the non inbred mate fits better you should really go for this. As I said a good selection is of the parents is more important then the mating system.
When mating related birds at least one of the parents must be non inbred. Preferably the other parent is inbred, but if you want to recombine the genes the other parent must also be non inbred.
The conclusion is that you have to balance inbreeding and mating related against each other.
I haven't talked about an often mentioned side effect of inbreeding and mating related: loss of fertility, loss of fitness, ... This is part of what is called inbreeding depression.
There have been many studies that look at the influence of inbreeding. There is an excellent book written about these studies: The Natural History of Inbreeding and Outbreeding (ed. Nancy Wilmsen Thornhill). It is not a book for the layman, so I'll try to touch upon the most important points raised in this book. The book is mostly about wild populations, but also talks about 'laboratory' experiments and zoo experiences.
First up a whole lot of studies about the effects of inbreeding just seem to be done badly. There seems to be some confusion on the definition of the term inbreeding, but the biggest error in a lot of those studies was to assume that pairs with unknown pedigree are unrelated.
Secondly the interpretation of the results seems to be difficult at times. A study done on the European bison in zoos first seemed to point to a significant decline in fertility because of inbreeding. Further examination of the data revealed that the lower fertility could all be traced back to one bull that turned out te be from a different subspecies. So the perceived inbreeding depression was actually outbreeding depression: compare it to the hybrid mule that is infertile.
A very nice study covering several years was done on the wild great tit in Holland. Following the population several years is essential for knowing the pedigrees. But also for something else: Pairs of related parents produced significantly less offspring then non related pairs. This seems to prove the inbreeding depression. But these related pairs had twice, yes twice!, as many grand children then the non related pairs. So their inbred offspring seemed very fertile and fit for producing offspring. It is being compared with investing in only a few chicks, giving each chick individually more attention as opposed to investing in quantity.
It is unclear how this study about the wild great tit is relevant to captive exhibition budgerigars, but it shows that inbreeding effects are a complicated matter.
Tests with various captive species (including budgerigars) specifically designed to investigate the effects of inbreeding have had mixed results: Some show some negative influence on fertility, other don't see any. The conclusion is that inbreeding can have a negative effect on fertility, but it doesn't necessarily have to.
In the end the conclusion of The Natural History of Inbreeding and Outbreeding was: it's inconclusive.
A statistical analysis of breeding records of several zoo species showed an overall negative effect of the kinship of the parents on the fertility(*) of a pair (). Using inbred females also seemed to be bad for fertility. But inbreeding further up the pedigree seems to have a slightly good influence on fertility. This is attributed to purging. Purging means that by inbreeding a gene bad for fertility is doubled up and subsequently culled because it creates infertility. The left over individuals in further generation consequently suffer less from this bad gene because of this culling.
An important note on this last study is that the effect of differences between species, breeding years and zoos each was far more important then the effect of inbreeding. There are also some other issues with the data but the conclusion is still interesting.
Crossing different species is even a clear examples of the opposite of hybrid vigor: oubreeding depression. Just think about the infertile mule.
First I want to repeat what I said earlier: A good selection of the breeding team is far more important than any breeding system.
Giving some thought on pairings based on inbreeding and kinship next to the visual compatibility can speed up the progress of improving your stud. Balancing inbreeding and kinship in pairs against each other certainly has its role in a well thought out breeding system.
On the other hand you just can't dismiss inbreeding depression. So you'll need to consider that too to some extend when pairing up. This is also one of the reasons why I prefer a pure Q4 mating over a pure Q1 mating.
Note however that inbreeding and kinship is a continuum from 0% to 100% and not a black and white feature: It is all relative.
Note: You dan find an article on calculating the inbreeding coefficient here (the kinship of two birds is the inbreeding coefficient of theoretical offspring from those birds).