Science has a way of nudging us out of our certainties. What once seemed fixed can, with the next experiment or field study, turn out to be more complicated. Few areas have shifted more dramatically than our understanding of sex in recent decades. The latest example comes not from humans but from birds.

In a new study in Biology Letters, scientists examined nearly 500 wild birds from five Australian species—kookaburras, magpies, crested pigeons, rainbow lorikeets, and scaly-breasted lorikeets. They compared each bird's genetic sex, determined by DNA markers, with its reproductive anatomy. The surprise was hard to miss; about 3 to 6 percent showed a mismatch. Genetically female birds (ZW) sometimes had male gonads, and in one extraordinary case, a genetically male kookaburra (ZZ) had a stretched oviduct and appeared to have laid an egg (Hall and colleagues, 2025). For birds—where sex chromosomes are the reverse of mammals, ZZ for males and ZW for females—it was a striking discovery.

This study shows that what scientists call the "three G's"—genes, gonads, and genitals—don't always match. When they don't, individuals fall into what scientists broadly call intersex. Humans are no exception, as the Princeton anthropologist........

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