New research on the glittering White-necked Jacobin hummingbird reveals nearly 20% of the species’ adult females have male-like plumage.
This strategy is all about dodging bullies and getting better access to food. The findings were published recently in the journal Current Biology.
As the birds mature, all the jacobin males retain the fancier plumage but so do nearly 20% of the females among the population Falk studied in Panama. The remaining 80% of females still develop the muted green and white coloration of a typical adult female. Though plumage ornamentation is usually attributed to sexual selection and attracting a mate, researchers ruled out that explanation for this species after field experiments.
Scientists observed the reactions of live jacobin hummingbirds toward stuffed mounts placed on nectar feeders during breeding season. The mounts were stuffed specimens of adult White-necked Jacobin males, typical adult females, and female adults that looked like males.
So, what’s the benefit to females of this species when they look like a male? To get to the root of that puzzle, Falk and his assistants put radio frequency ID tags on birds and set up a circuit of 28 feeders wired to read the tags. By tracking the number and length of visits, they honed in on the answer.
The researchers found that the male-like females got to feed longer than the typical adult female—about 35% longer at feeders filled with high-sugar nectar. That can make a big difference because hummingbirds have the highest metabolic rate of any vertebrate. They need to eat constantly in order to survive.
Bottom line: female White-necked Jacobins retain the male-like plumage of their youth for social reasons—they avoid the bullies by looking like them. It is still not clear whether male-like females behave just as aggressively as the males. The actual physical mechanism that allows females to retain male-like plumage is also not known.
The White-necked Jacobin is hardly alone when it comes to having some females that look like males. Falk says studies have found that 25% of the world’s 350+ hummingbird species also have some females that look like males.
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Illustrations, from above: White-necked Jacobins by Brian Sullivan; and left and center images show adult female and adult male plumages, respectively, right image shows juvenile plumage by Jillian Ditner, Cornell Lab of Ornithology.