Red heads don't mix with Black heads (Photo Sarah Pryke)

Yesterday I came across a new article on the ABC web site with some very interesting news for Gouldian breeders.  If you have subscription to  you can read the actual academic article too from Sarah R. Pryke and Simon C. Griffith (2009) Genetic Incompatibility Drives Sex Allocation and Maternal Investment in a Polymorphic Finch. Science , 20 March 2009: Vol. 323. no. 5921, pp. 1605 – 1607.  I have a PDF file of this if anyone is particularly interested then contact me and I can send it to you.

According to Sarah “Over eighty per cent of Gouldian finch chicks will be male if their mother sees that the father has a different coloured head”. The allocation of sex is determined by what the female sees and is not due to any chemical or genetic interaction between the parents. Experiments to change the colour of the male’s head with hair dye (the research was sponsored by L’Oreal) and the female will alter the sex ratio within the clutch. “This discovery will change our understanding of sex determination across the animal kingdom,” says Sarah.

“Gouldian finches wear their genes on their head.  These colourful native birds have three possible head colours-red, yellow and black. So it was relatively easy to construct an experiment to determine the influence of head colour on sex.”

“We expected some influence on sex ratio,” she says, “but it was a surprise to find that 82.1 per cent of the offspring were male.”

Why is it so? “Daughters produced from mixed matings-where parents differ in head colour – suffer from genetic incompatibilities between their parents that cause about 84 per cent to die young,” says Sarah.

So it’s in the finch’s best interest to mate with a male with the same head colour. If a female has no other option but to mate with a male of different head colour, she will produce mainly sons to maximise the chicks’ chances of survival. Since the Gouldian is now a rare and threatened species in the wild this is extremely important.

Sex determination birds is determined not by the random allocation of a “Y” chromosome from the male as in mammals but in birds there is selective allocation of  the “W” or “Z” chromosomes by the female to the developing oocyte during meiosis.  This gives birds much greater flexibility to control populations and the expression of genetic disorders.  This is akin to selective abortion of male embryos in humans when there is a chance of inheriting a disorder like haemophilia or colour blindness.  Birds do this prezygotically and so don’t risk the condemnation of the church!

This capacity also probably explains why captive finches produce excess males.  Food is usually a limiting resource in the wild but in captivity the limiting factor is space.  If a female were to produce more females the situation would become worse but by raising more male offspring she, subconsciously, allows the male offspring to fight it out for the few females available in order to raise the fitness of her descendants.