There are two aspects here. There is the important effect of fertility on selection but I will warm up by discussing selection for better fertility.
Fertility is known in many species to be a difficult trait to select on. In expert-speak: a low heritability. The success in mice experiments had varying results. Now there certainly is a genetic component to fertility. It's no accident that birds from certain studs are difficult to breed. So it certainly is possible to select on fertility. But do you want to select on fertility? There are so many other traits to consider when selecting. Do you really want to favour a lesser hen with superior fertility over a better hen? After all fertility is not a show quality. I will get back to that in the second part of this article. For now lets assume a certain level of fertility in your stud is desirable.
Since fertility has such a low heritability it is very important to look at the breeding records of ancestors and other relatives. Fertility is a very complex trait and is strongly determined by outside influences. For instance if a hen is disturbed at night and has difficulties finding back the nest then the eggs will have cooled and the hatching rate will be lower. Another example: badly attached perches in the breeding cage will result in more clear eggs. So when selecting on fertility it is very important to keep the environment as constant and uniform as possible. You can't compare the fertility of a pair breeding in winter with a pair breeding in summer when they are housed in an outside aviary. Temperature, lighting intensity, hours of sunshine a day, rain, ... is different.
To summarize, keep records and heavily use information of relatives and have an environment (housing, feeding, management routine, ...) that is as uniform as possible.
Fertility is not a show quality, so do we need it in our stud? Everyone will agree that a minimal fertility is required: on average you need to breed two children from each parent just to save your stud from extinction. But with this minimum you can't cull, so there is no selection. From the theory of selection you can get an idea on what the quality will be of the best chick if the "average" chick has a quality of 0 (this is statistics, so these are averages over a whole lot of pairs):
| Number of chicks: | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| Quality of the best chick: | 0 | 43 | 67 | 84 | 97 | 107 | 115 | 122 | 128 | 134 | 138 | 143 | 147 | 150 | 153 | 156 | 159 | 162 | 164 | 167 |
I won't bore you with the mathematical and statistical background of these numbers, but let's just demonstrate intuitively: Say you breed 4 chicks from a pair. From this pair one of the 4 is the best. Suppose you suddenly discover you moved an egg from this pair and that you've got a 5th chick. This 5th chick can be better or worse than the best one of the 4 others. If it's better, then the quality of the best chick from 5 is higher than that of 4 chicks. In case the 5th chick is worse, the quality of the best chick from 5 is equal to the best chick from 4. So you can only win if you breed more. Which means that statistically, with a great number of pairs, that extra chick per pair will get you better quality.
Now intuitively you can understand that the chance that this extra chick is better than all the previous ones is smaller when there are more previous chicks. This is also reflected in the numbers above. The more chicks you breed, the less effect that extra chick per pair has. For instance chick 20 gives an increase of 3 points (from 164 to 157), while chick 5 gives an increase of 13 points (from 84 to 97).
Breeding more chicks allows you to be more rigorous in culling. So you'll have better parents next breeding season. And this then reflects in better chicks next year.
But you have to balance the benefits and the costs. If you already breed 10 chicks per pair over two rounds you probably don't want to select on fertility. The extra effort will not be proportional to the benefits. But if you breed 3 chicks per pair getting the fertility up will allow you to make bigger steps each year. The difference between 3 (67 points) and 6 (107 points) is huge.
The choice as always is yours, but I hope I've helped you in making a more informed judgment.
(C) Bert Raeymaekers