Which bird has four sexes?

No bird has four biological sexes in the way mammals have male and female, because birds have two sexes, male and female, determined by Z and W chromosomes, opposite to mammals, but one bird species is famous for having four distinct morphs or forms that behave like four sexes due to complex genetics and mating behavior. That bird is the White-throated Sparrow, Zonotrichia albicollis, which has two color morphs, white-striped and tan-striped, and these morphs exist in both males and females, resulting in four combinations, white-striped male, white-striped female, tan-striped male, and tan-striped female. These morphs have different behaviors, white-striped birds are more aggressive and promiscuous, tan-striped birds are more nurturing and monogamous, and they almost always mate with the opposite morph, white with tan, which maintains the polymorphism. Because of this disassortative mating and behavioral differences, scientists sometimes describe them as having four sexes, though biologically they are still male and female with two morphs. Another bird with similar complexity is the Ruff, Philomachus pugnax, which has three male morphs with different mating strategies. So while no bird truly has four biological sexes, the White-throated Sparrow is the bird most commonly described as having four sexes due to its two morphs in each sex that function like four mating types.