The white-throated sparrow (Zonotrichia albicollis) is a North American songbird with a very unusual mating system. The species exists in two color morphs: white-striped birds with bright black and white head stripes, and tan-striped birds with duller brown and tan stripes. This difference is caused by a genetic change called a chromosomal inversion, sometimes described as a “supergene.”

Both males and females can occur in either morph, which creates four types of birds: white-striped males, white-striped females, tan-striped males, and tan-striped females. Most pairs form between opposite morphs.

The morphs also behave differently. White-striped birds are usually more aggressive and territorial, while tan-striped birds tend to focus more on feeding and caring for young. Because of this, pairs often split roles between defense and parental care.

This system is sometimes informally described as “four sexes.” While it is not the same as human gender identities, it is sometimes mentioned in discussions about biological diversity, including ideas such as nonbinary or agender identities, to illustrate that sex and behavior in nature can be more complex than a simple two-type system.

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