As in humans and many other species, biological sex in birds is binary. Males and females are distinguished by their gametes, the cells that fuse with each other during fertilization in the biological process of sexual reproduction.
However, there are some important and intriguing differences in the way the sex chromosomes inside the cells of birds influence the characteristics of the offspring produced.
How do male and female birds develop?
In humans, the DNA of each cell normally contains 23 pairs of chromosomes. 22 of these are the same in both males and females but the 23rd pair differs, and it is this pair that determine sex.
In the last pair, girls and women have two copies of a large gene-rich chromosome called X, whereas boys and men have one large X chromosome and a tiny Y chromosome. There are some exceptions, of course. People born with a group of rare conditions called differences in sex development (DSDs) may have a differing number of sex chromosomes, but in the same way that humans are said to be bipedal, even if some people are occasionally born with only one leg, homo sapiens as a species has two distinct sexes – male and female.
The XX/XY sex-determination system is found in most other mammals as well as in some insects, and the sex of a species in which the sex chromosomes are not the same – in the case of mammals, the male – is referred to as the heterogametic sex.
Birds have approximately 80 chromosomes which also includes a pair of sex chromosomes. However, in birds they are the opposite way round from humans. Male birds have two copies of a large gene-rich chromosome called Z while female birds have one Z chromosome and a smaller W chromosome, and they are the heterogametic sex.
In humans, a gene on the Y chromosome is responsible for initiating the development of the male gonads (testes) in an embryo. The testes make testosterone, a hormone which plays a key role in the development of male characteristics such as the penis, prostate, and scrotum, as well as body hair, a deeper voice, fat distribution, and increased muscle and bone mass.
Testosterone also suppresses the release of oestrogen responsible for the development of breasts and wider hips found in women. If the group of cells that will eventually turn into gonads don’t get the signal from the Y chromosome and grow into testes, they will eventually become the female gonads or ovaries. In other words, in humans, female is the default sex.
We know for sure that it’s the Y chromosome in humans that’s responsible for determining sex, because people with a single X chromosome (XO) are women who can become pregnant, while people with XXY are men, and are capable of fathering children.
However, unlike in humans, no birds have ever been discovered that have a difference in the number of sex chromosomes and therefore we can’t be sure which chromosome it is that determines the sex of a bird.
One hypothesis suggests that a gene on the Z chromosome, called DMRT1, determines the sex of a bird embryo. In a ZZ embryo the two copies of DMRT1 start the process of testes development which produces testosterone, whereas in a ZW embryo the single DMRT1 causes the gonad to develop into an ovary and produce a female bird.
Another hypothesis says that it’s the W chromosome that kicks off ovary development and inhibits testes development. Although the W-linked gene hasn’t been identified spontaneous sex reversal in birds strongly supports this hypothesis.
Can birds change sex?
In some species of birds, notably ducks, chickens, pheasants, and peafowl, females have been observed developing secondary male characteristics such as bright plumage and ornamental feathers. It is an occurrence that has intrigued the scientific community for centuries. In 1888, John Henry Gurney Jr. a British ornithologist, published On the Occasional Assumption of the Male Plumage by Female Birds in The Ibis, the peer-reviewed scientific journal of the British Ornithologists’ Union and he referenced a paper that referred to the phenomenon written in 1780.
A male bird’s characteristics like showy plumage, singing voice, size, and dancing ability are controlled by his Z chromosome. Female birds also have a Z chromosome but it is prevented from producing testosterone by oestrogenwhich inhibits the gene that would unusually trigger male characteristics.
However, although two ovaries are present in embryonic female birds, by the time they reach adulthood only the left one is functioning. With no spare, if it gets damaged through disease, injury, or old age, then the production of oestrogen ceases, there is nothing to suppress the release of testosterone, and a female bird may start to assume secondary male characteristics, known as gynandry.
There is no documented evidence of male birds developing female sexual characteristics which is why the observation supports the hypothesis that male is the default sex of birds, and the W chromosome is responsible for the development of females.
Some people refer to gynandry as a sex-change but this is incorrect. A female bird may have the appearance of a male, but she has not actually transformed into a male. She still has her ZW chromosomes and although she will have stopped laying eggs other male birds may still try and mate with her.
It is not known how many birds develop gynandry in the wild since the identity of the female birds is not known. Many of these birds become lethargic during the male moult and are easy pickings for predators so do not survive long enough for continuous observation. In domesticated poultry which are bred for their high egg production, the likelihood of ovarian damage is increased which is why gynandry appears to be much more prevalent in captive birds.
What about birds that are both male and female?
Another interesting but rare abnormality called gynandromorphism has also been observed in birds. This is when they take on both male and female characteristics sometimes with spectacular results as seen in this half red and half brown northern cardinal spotted by Jamie Hill in Pennsylvania in 2021..
Gynandromorphism has never occurred in humans that we know of but has been seen in spiders, butterflies, lobsters, bees, and many species of birds. It is most obvious in animals that display high levels of sexual dimorphism.
It is not fully understood why gynandromorphism happens in some organisms, and may occur in a variety of different ways. In birds most likely explanation is from the fusion of two separate ZZ and ZW embryos, or when a female (ZW) egg cell is fertilized twice by two Z-carrying sperm.
In bilateral gynandromorphism the body is perfectly divided exactly down the centre with one side female and the other male. If you were to examine a cell from the female side it would contain ZW chromosomes while the cells on the male side would contain ZZ chromosomes.
Usually, gynandromorphs are infertile but if the left-hand side of the bird is female – the side of the functioning ovary – as in the cardinal shown above, then it may be able to reproduce.