Science

...In humans, individuals with XX chromosomes typically develop as female, whereas those with XY chromosomes are usually male. But Judith Mank, a zoologist at the University of British Columbia, notes it’s the genes carried on those chromosomes—not the chromosomes—that are the main players. The SRY gene on the Y chromosome, for example, kick-starts male development in mammals. Anyone missing this key gene will end up developing as female, even if they have XY chromosomes. “We think of sex chromosomes as being sex determining,” says Mank, who also wasn’t involved in the new research. “That’s not true”...

Environmental factors can also complicate sex determination. Turtle eggs that incubate at cooler temperatures, for example, produce male hatchlings, whereas turning up the heat churns out females. Although rare in birds, this phenomenon has been seen in brush turkeys, which incubate their eggs in enormous mounds....

[The study in Biology Letters] team was surprised to find sex-reversed individuals in all five species, at rates of 3% to 6%. Nearly all these discordant birds were genetically female but had male reproductive organs. However, the researchers also found a few genetic males with ovaries—including a genetically male kookaburra with a distended oviduct, indicating it had recently laid an egg.

The findings suggest sex reversal is more common in wild birds than previously thought, which may have implications for the conservation of threatened species. It’s often important to know how many males and females there are in a population, because sex ratios influence the group’s ability to reproduce, grow, and sustain itself. “If you’re going off the genetic sex ratio,” Potvin says, “you may end up being surprised.”...

https://www.science.org/content/article/sex-reversal-surprisingly-common-birds-new-study-suggests

The ability to unequivocally identify the sex and reproductive status of individuals is crucial across many fields of study. Recent evidence indicates that avian sex determination is more flexible than once believed, with sex-reversed individuals identified in domestic bird populations—that is, individuals exhibiting gonadal and morphological traits of one sex while possessing the chromosomal make-up of the opposite sex.

The presence of these individuals can challenge the reliability of traditional sexing methods that rely solely on external morphology, internal anatomy or genetic markers. These methods, when used in isolation, fail to identify sex-reversed or intersex individuals, potentially overlooking their impact on population dynamics.

In this pioneering study [?as modestly determined by OURSELVES ], we investigated the prevalence of sex-reversed individuals in five common free-living avian species in Queensland, Australia. By comparing internal and external morphological characteristics with polymerase chain reaction results from sex-linked molecular markers, we identified sex-reversed individuals in all five species, with rates ranging from 3 to 6%.

Our findings suggest that sex reversal is a common and potentially widespread phenomenon in avian species.

https://royalsocietypublishing.org/doi/10.1098/rsbl.2025.0182