The long-running mystery of why birds seemingly change sex

It's not often you do a search on a scientific subject and come up with a journal from 1888, but people were puzzled by this mystery for a while: Female birds, usually once they've reached a sufficiently advanced age, can suddenly grow male plumage and take on male behaviors.

I read in a book that, seemingly at random, female birds could start growing male plumage. Intrigued, I did a quick search for why, and got a journal article entitled, "On the Occasional Assumption of the Male Plumage by Female Birds," which dated back to 1888. That article, which I found a little old-fashioned, started out by sniffing at the quaint old-fashioned conceits of another article written in 1780. Obviously, this question has gone back a long time, puzzling many scientists.

Some of these early scientists were confused by a red herring — the fact that sometimes female birds developed patches of male plumage. Some birds were completely divided, one half of them being male and one half of them being female. This condition, known as bilateral gynandromorphism, is the result of genetic mosaicism. For whatever reason, a clump of cells with male chromosomes gets attached to a clump of cells with female chromosomes and knit together into one animal. This gave scientists a lot of information that unfortunately was completely misleading. The sudden appearance of male plumage on female birds is not genetic at all.

When people began dissecting female birds that suddenly displayed male plumage, the found atrophied or nonfunctioning ovaries. When they removed the ovaries from healthy female birds, many of them began developing male plumage. Estrogen is the only thing that keeps female birds camouflaged in browns and grays. As ovaries age and stop working, or if they for some reason sustain damage, the sudden loss of estrogen causes females to grow vivid plumage, and even start calling or crowing like male birds. This is most often observed in chickens — since they're the most commonly observed birds — but it's been shown to happen in golden pheasants and even peacocks. As females peahens age, they will often grow full peacock tails, the way male peacocks do, although they won't fan them. We think of male plumage as an extravagance that is added to birds. It seems as if it's more of a default that needs to be hormonally suppressed to keep from showing up.

Image: Becks

Via Ibis, The Biological Bulletin, Bird Research, Feather Site.