A study of five common Australian bird species in southeast Queensland has found on average, 4.8 percent of them have sexual organs that don’t align with their chromosomes. The authors don’t know the cause of the deviation from biology textbooks, nor how representative the findings are for avian populations elsewhere, but it does appear to be more common than thought in wild birds. The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

Birds usually determine sex through a ZW chromosome system . Instead of two XX chromosomes making a female and XY making a male, as in most mammals, ZZ birds are male and ZW birds are female, or at least that is what aspiring biologists learn.

Just as multiple exceptions exist in humans in the form of Swye

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