Female plumage polymorphism is rare in hummingbirds

In dimorphic hummingbirds (Trochilidae), females exhibit variation in extent of male-like ornamentation. Female Anna’s Hummingbirds (Calypte anna) show substantial individual variation in the number of pink, male-like gorget feathers. Here, on a sample of nearly 500 female Anna’s Hummingbirds recaptured up to 11 years after banding, we show that gorget size increases substantially when the birds molt from immature into adult plumage. Thereafter, gorget size barely increased with age, meaning that age explains little of the individual variation in number of male-like feathers once a female is adult. A recent study suggested that female polymorphism is widespread in hummingbirds. We reassessed 13 species and show that, relative to adult females, the other “morphs” are actually: immature females; mis-sexed immature or adult males; products of inaccurate field-guide depictions of plumage; an invented morph; subspecific plumage variation; or the conflation of statistical bimodality with polymorphism. We conclude that, although many species of hummingbird show high variation in female plumage, true female polymorphism has only been convincingly demonstrated in perhaps three species (0.9% of hummingbirds). The most striking example is the White-necked Jacobin (Florisuga mellivora). This species appears unique in that adult, breeding females come in two morphs, one that is essentially indistinguishable from the brightly colored males, and the other that is dull and lacks iridescent feathers.