As regular CFZ-watchers will know, for some time Corinna has been doing a column for Animals & Men and a regular segment on On The Track... particularly about out-of-place birds and rare vagrants. There seem to be more and more bird stories from all over the world hitting the news these days so, to make room for them all - and to give them all equal and worthy coverage - she has set up this new blog to cover all things feathery and Fortean.

Friday 22 March 2019

How birds become male or female, and occasionally both


March 11, 2019 by Jenny Graves, The Conversation
The highly unusual "semi-identical" Australian twins reported last week are the result of a rare event. It's thought the brother and sister (who have identical genes from their mother but not their father) developed from an egg fertilised by two different sperm at the same moment.
In humans, it's the sperm that determines whether an embryo is pushed along a male or female development pathway. But in birds, it's the other way around. Eggs are the deciding factor in bird sex.
There are other fascinating aspects of bird sex that are not shared with humans. Female birds seem to have some capacity to control the sex of their chicks. And occasionally a bird that is female on one side and male on the other is produced – as in recent reports of this cardinal in the United States.
X and Y, Z and W chromosomes
So what is it about bird chromosomes that makes bird sex so different from human sex?
In humans, cells in females have two copies of a large, gene-rich chromosome called X. Male cells have one X, and a tiny Y chromosome.
Birds also have sex chromosomes, but they act in completely the opposite way. Male birds have two copies of a large, gene-rich chromosome called Z, and females have a single Z and a W chromosome. The tiny W chromosome is all that is left of an original Z, which degenerated over time, much like the human Y.
When cells in the bird ovary undergo the special kind of division (called "meiosis") that produces eggs with just one set of chromosomes, each egg cell receives either a Z or a W.

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