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By observing Jacobin white-necked hummingbirds in Panama, researchers found that a large proportion of females sported the same brightly colored ornamentation as males; by pretending to be one of them, they avoid the aggressive behavior of their male counterparts during feeding or mating. In fact, all of the juvenile females displayed typical male shimmering colors, which is very unusual for birds.
Sexual dimorphism is generally very pronounced in birds. In a large number of species, the males have more showy colored feathers, while the females are more discreet and calm. Scientists previously thought that the same was true for the Jacobin hummingbird (Florisuga melivora), but a new study reveals that, on the contrary, the females of this species adorn themselves with bright colors to resemble the males.
This unusual behavior seems linked to a need for protection. Indeed, to assert their dominance, males adopt aggressive behavior towards females (pecking, plucking of feathers, body shocks), especially when feeding. Mimicking the plumage of their congeners would thus protect the females from these untimely attacks.
A plumage that facilitates access to food resources
But the researchers were also surprised to notice that all the young females had very colorful plumage, which is very unusual. Indeed, when adult males and females have marked sexual dimorphism—in other words, when they display many morphological differences—juveniles generally resemble adult females (and not males). But that was not the case here: the young hummingbirds all sported very bright plumage, just like the males.
The latter have feathers of bright colors and rather flashy, the iridescent blue head, the belly and the tail of a particularly brilliant white. Female Jacobin hummingbirds, on the other hand, tend to be much less showy, with green, gray or relatively dark colors, which allow them to blend in with their surroundings.
Jay Falk, an ornithologist at the University of Washington, and his team found that nearly 20% of adult females retain the bright colors they’ve had since birth — while the remaining females ‘tarnish’ as they age . But to date, specialists have not managed to determine the exact cause of this phenomenon. This can be due to a genetic factor as well as an environmental factor.
Through an experiment, Falk and his colleagues still managed to identify the role of this plumage. They set up stuffed hummingbirds on feeders, then watched real hummingbirds interact with them depending on their coloring. They then discovered that hummingbirds mainly harass dark-colored females, suggesting that showy colors are caused by social selection: plumage is a way of escaping the harassment that most female hummingbirds experience. In addition, because they suffered less attack, androchromic females (with the same plumage as males) had greater access to feeders than heterochromic females.
A factor of social and not sexual selection
Ornamentation is often explained by sexual selection; indeed, in many species, physical attributes evolve during courtship, for example. But the researchers note in their study that social competition for resources — whether used for reproduction or for non-reproductive reasons — can also promote the evolution of elaborate traits, especially in females. That’s what Falk and his team proved here.
Another observation supports this hypothesis: most females displayed showy colors during their juvenile period and not during the breeding period. In other words, the only time they display colorful feathers is precisely during the period when they are not looking for a mate. Which argues that this particular plumage has no role in sexual selection. Especially since during the experiment, in 100% of the trials, the first sexual advance was made towards a heterochromic female.
In future studies, Falk and her team hope to use the results of observed variation between white-necked Jacobin females to understand how this variation between males and females may evolve in other species.