Female Birds Can Bias The Sex Their Chicks

Whether a bird is more likely to lay a male or female egg depends on which sex will have the greatest chance of doing well. Rutstein et al. (2004) adjusted the food intake of female Zebra Finches [see photo of female (left) and male (right) Zebra Finches below right] & found that well-fed females were more likely to produce daughters, while less well nourished birds were more likely to have sons. This is exactly as predicted by the fact that female offspring need to be better nourished than males if they are to survive and grow well.
The authors noted that: “In most animals sex ratio is close to 50:50 and extremely resistant to change. In mammals, including humans, the sex of the baby is determined by whether the sex chromosome in the sperm is male or female. But in birds, it is the female’s egg rather than the male’s sperm that determines what sex the chick will be. Thus the female has the potential to determine the sex of her young by whether she ovulates male or female eggs. In some way, female Zebra Finches seem to be able to exert control over whether to produce a male or female egg depending on which of the two is most likely to be successful. Our research tells us that they do it, and we understand why. The big question is: how do they do it?”
In many animals, females need to be well-nourished and in good condition if they are to breed, as eggs are costly to produce. Bigger eggs tend to lead to bigger young that are more likely to survive. Such ‘sex ratio adjustment’ is well documented in certain insects, such as bees and wasps, but is less well understood in birds and mammals.
Birds are an excellent model to use in the study of sex ratio adjustment because, using molecular techniques, scientists can establish the sex of each egg soon after laying. Further, all the resources given to the developing embryo are present in the egg at laying. Thus the size and the content of the egg are measures of the amount of resources that the female has allocated to that egg, which affects its subsequent survival chances.
The authors explained: “We manipulated the diet quality of Zebra Finches to look at the effects of body condition on female investment. We found that females were able to exert a strong degree of control over the production of male and female eggs. When females were fed on a low quality diet, they laid eggs that were considerably lighter than those laid when they were fed on a high quality diet, and they also laid far more male eggs on a low quality diet. This is the converse situation to that described 30 years ago for mammals, but it makes sense for Zebra Finches. Previous research has shown that under poor nutritional conditions, female Zebra Finches grow more slowly and survive less well compared to males. Therefore, females are producing more of the sex with the highest survival chances under those conditions.”

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Two potential mechanisms for determination among birds. (A) the presence of the W chromosome triggers femaleness or (B) the presence of two Z chromosomes confers maleness.

Avian sex determination (Ellegren 2001) -- The molecular determinants behind sexual development in birds remain a mystery. The process is known to be different from that in mammals, with no homolog to the gene that confers maleness in mammals found in birds. The failure to identify such a gene in birds is probably a reflection of the fact that, despite the occurrence of two sexes being nearly universal throughout the animal kingdom, the genes involved seem virtually unrelated among metazoan phyla. These differences raise obstacles for comparative or candidate gene approaches in studies of sexual development. In birds, females are the heterogametic sex, with one copy each of the Z and W sex chromosomes. Males are homogametic (ZZ). However, it is not clear whether it is the presence of the female-specific W chromosome that triggers female development, or the dose of Z chromosome that confers maleness. An intriguing additional possibility is that both Z and W matter! In marsupials, for example, Y acts as a dominant testis determining chromosome, while the X chromosome determines the choice between pouch and scrotum. Maybe a system where the two sex chromosomes mediate different aspects of sex differentiation is also used in birds.

Vertebrate sex determination systems. Phylogeny of major vertebrate clades showing the sex determining systems
found in members of the respective clade. ‘Multiple’ indicates involvement of more than one pair of chromosomes in sex determination.
TSD: temperature-dependent sex determination (From: Ezaz 2006).

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Incubation temperature and avian sex ratios -- Although common in reptiles, incubation temperature has not been considered to be a factor in determining sex ratios in birds. However, Goth and Booth (2005) found that incubation temperature does affect sex ratios in megapodes, which are exceptional among birds because they use environmental heat sources for incubation. In the Australian Brush-turkey (Alectura lathami), a mound-building megapode, more males hatch at low incubation temperatures and more females hatch at high temperatures, whereas the proportion is 1:1 at the average temperature found in natural mounds. Chicks from lower temperatures weigh less, which probably affects offspring survival, but are not smaller. Megapodes possess heteromorphic sex chromosomes like other birds, which eliminates temperature-dependent sex determination, as described for reptiles, as the mechanism behind the skewed sex ratios at high and low temperatures. Instead, Goth and Booth (2005) suggest a sex -biased temperature-sensitive embryo mortality because mortality was greater at the lower and higher temperatures, and minimal at the middle temperature where the sex ratio was 1:1.


http://people.eku.edu/ritchisong/avianreproduction

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