The Wise Owl

Beating the Odds—The Future of Breeding Management

Words and Photographs by Dr Milton Lewis BSc (Hons) PhD

Have you ever had those moments when you are sorting through the young birds of the previous season and despaired at the number of cocks? What about the times when you have bred with your best pair, produced plenty of good quality young and found that there is only one hen to keep the line continuing? I am sure it is a problem that most of us have encountered yet thought there was little that could be done to improve the situation.

What about those species that are becoming a little difficult to find in Australian aviaries and suffer from occasional breeding seasons where young of only a single sex are produced? The Green Avadavat is one such example, however there are many more that have declined in numbers over the years, sometimes much faster than expected because of having too many young of one sex.

On occasion I have mentioned that Zebra Finch hens are able to manipulate the proportion of cocks and hens that fledge from their nests. This has always been a favourite topic of mine but I have mostly skimmed over the details. The ability of birds to manipulate the ratios of cocks to hens that fledge from their nests is perhaps more widespread than we think and certainly not just confined to Zebra Finches.

Several studies have investigated this aspect in wild birds and there is some very solid evidence for the phenomenon. An Australian example is the Red-capped Robin—the results from a study of this little bird are quite astounding. Briefly, it was discovered that older hens in this species are more likely to fledge daughters. This is compared to young hens in their first breeding season which raise more sons. This species also has a rather high level of extra-pair paternity—ie the number of young whose genetic fathers are not the cocks that feed them in the nest. In these nests, fledglings are predominantly cocks. Obviously there is a great deal going on in the lives of these species giving them reason to make choices that we as mere humans are only just beginning to understand.

So how can this help us breed a more even number of both sexes in our aviary birds? Well, we know that birds are able to manipulate the sex ratio of their surviving young. We now need to look at the mechanisms and reasons for birds doing such a bizarre thing.

The maternal condition hypothesis is a theory used by behavioural ecologists to explain the phenomenon of differing nestling sex ratios. This theory makes the prediction that mothers should choose offspring sex according to their own body condition. Hens, for example, need to think carefully about the effort they exert in raising sons or daughters depending on how well they have managed to pack on body condition before egg laying.

Wild birds like this Red-capped Robin need to make tough choices about which of their nestlings will survive when food and living space is in short supply

However, there is a very important underlying assumption—that one sex has to be more variable in reproductive success. This is the main reason why hens should choose between raising sons or daughters. Generally it is considered that cocks of most species display the most variance in producing offspring. We all know that some cocks are complete losers and never reproduce, whereas some studs are fathering young all over the place. On the other hand, hens just about always have several offspring so their reproductive output is not as variable.

In simple terms it appears that hens in very good body condition generally raise more cocks than hens. Hens in poorer body condition or older age hens tend to raise more daughters. These observations fit well within the theory when you think about the options a hen is facing. A hen in poor condition is unlikely to be able to raise a whole clutch and, if her condition is very low it may be her only clutch this season or, even worse, the only clutch in her life. In this case she would probably choose to raise the hens that have hatched. No matter how healthy or sexy her daughters, they will almost always produce young if they survive to breeding age. Sons, on the other hand, are unlikely to be as healthy as those produced by mothers in better body condition, and are probably going to be less attractive and therefore never find a mate.

A hen that has been able to obtain high quality food before egg laying and is therefore in good body condition has a very different set of decisions. This hen has the option of raising all her young if conditions remain good. In addition, because she has been in good condition she has increased both the number of eggs in her clutch and the average size of these eggs. These hens also mate with the highest quality mates so they are more likely to produce sons of high quality. Producing healthy attractive sons will, in the long term, be much better for ensuring a large number of grandchildren. Sexy sons have a high chance of mating with lots of hens in a single season and fathering multiple clutches of young, representing high reproductive success.

How hens actually manipulate the sex ratio of nestlings is a bit of a puzzle, however there is some evidence that the selection is made soon after hatching. It is not often that hens of any species raise all the chicks that have hatched. The chicks they do raise have been preferentially selected, possibly because of sex.

I know that all of this may sound a little confusing and perhaps a bit far fetched, but let me assure you that the science behind these ideas is rigorous. We have known for many years that the sex of turtle eggs depends on incubation temperature, so it should not be such a stretch to believe that birds really do adjust the sex ratios of their clutches depending on their health, ie body condition and the attractiveness or quality of their mates.

We should be able to use this to our advantage in aviaries. Theoretically, we could increase our chances of breeding hens or cocks by adjusting the quality or quantity of food for our breeding stock. After reviewing the breeding records of my Zebra Finches for the past 10 years there is quite obviously a trend in my aviaries for more cocks than hens. Previous evidence suggests that both young hens and well-conditioned hens are most likely to raise sons. We all take great pride in how well we feed our birds before the breeding season, but I am now considering a few more adjustments in the preparation schedule of my hens. Perhaps less supplementation is in order, but it will take a lot more thought before I completely change their diet.

Breeding plenty of young cocks seems to be reasonably simple - keep young hens and feed them very well during the pre-egg laying period. There is also the option of adjusting the attractiveness of cocks by adding coloured rings that my enhance their sexiness. To achieve this with Zebra Finches it is a simple matter of applying red leg rings.

The task of breeding a great number of hens is a little trickier. There is a trend for my older hens to produce more daughters than younger mothers, which is just as the theory predicts. Unfortunately, however, I usually keep majority of younger hens which leads to the breeding of more cocks. Obviously there are two choices for breeding more hens - keep a higher percentage of older hens, or feed a poorer diet - perhaps with fewer supplements - a few weeks prior to egg laying. I think it will be necessary to trial a few different diets before I can confidently manage this part, but it gives me something more to try over the next few years.

As always there is more to the story, but I think you will agree it is an interesting topic and certainly food for thought. Imagine if we could take our breeding management to this next level and be confident that every year would provide us with both enough hens and cocks for continued success in our hobby. I like to keep an open mind, so I will be pursuing this topic further in the coming breeding seasons. The future of our hobby relies on improvement and innovation. Good luck with the next breeding season.

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