"If you're not confused, you're not paying attention."
― Tom Peters
Some birders (and researchers) describe the White-throated Sparrow (Zonotrichia albicollis) as having four sexes, but that's a catchy oversimplification. Biologically, these birds still have only two sexes: males that produce sperm and females that produce eggs. What sets them apart is an interesting genetic twist — a chromosomal inversion, often called a supergene — that produces two plumage and behavior types: white-striped and tan-striped. The sex-determining system (ZZ/ZW) and the morph supergene (ZAL2/ZAL2m) are completely separate genetic mechanisms.
(click to enlarge)
Almost every (but not all) breeding pair is made up of one bird from each morph, so in practice you see four combinations (white male, tan male, white female, tan female). But those aren't four biological sexes — they're two sexes expressed in two morphs. The real fascination lies in how this unusual system enforces outbreeding and maintains genetic diversity, not a redefinition of sex binary.
Suggesting that the White-throated Sparrow somehow proves human categories of sex are outdated isn't biology — it's what philosophers call the Reverse Naturalistic Fallacy: taking an oddity in Nature and using it as evidence for a social or moral claim about people. It doesn't overturn the basic fact that these birds reproduce through two sexes.
All images © 2025 Mike McDowell