Usually female white-naped hummingbirds wear inconspicuous camouflage colors, but some of them deviate from this and have as variegated plumage as the males. Researchers have now found out what is behind it. Accordingly, the driving factor in this case is not sexual selection, because the males clearly prefer their camouflaged partners. Instead, tests revealed that females that look like males are less likely to be victims of aggressive conspecifics. This gives them better access to food than typically female colored individuals.
The colorful plumage of many birds is often the result of sexual selection: those who look particularly gorgeous have the best chances with the opposite sex. Usually it is the males who are brightly colored, while the females have more camouflage colors. So they are better protected during the brood. For the same reason, young animals are usually colored rather inconspicuously. Only with sexual maturity do the males develop their magnificent plumage.
However, it is different with some species of hummingbird, including the white-naped hummingbird, which is common in South and Central America. “The interesting thing about the white-naped hummingbird is that all young animals have male-looking plumage at the beginning,” says Jay Falk of Cornell University in New York. Some of the adult females are also male in color. Together with his colleagues, Falk got to the bottom of this phenomenon. To do this, the researchers first caught 436 wild white-naped hummingbirds and documented their color, gender and age.
Aggression against females
The result: around 20 percent of all adult females, like the males, have a bright blue head and white breast. The remaining 80 percent are colored a more inconspicuous green. The young animals have the apparently masculine colors regardless of gender. Next, the researchers tested how the white-naped hummingbirds react to differently colored conspecifics. To do this, they put stuffed males and females at feeding points and watched how the real hummingbirds interacted with them. “The hummingbirds were much more aggressive towards typically colored females than towards males and male colored females,” the researchers report.
In all experiments, sexual behaviors were also initially related to the typically inconspicuous colored females. “If the male-looking plumage of the females were the result of sexual selection, then the males should have been more attracted to females with male plumage,” says Falk. “But that didn’t happen.” Instead, the male white-necked hummingbirds showed a clear preference for inconspicuous camouflaged female partners Sexual Selection Hypothesis.
Masculine appearance as protection
Instead, the conspicuous plumage apparently serves to protect itself from being harassed by conspecifics. The researchers tested this with two further experiments. On the one hand, they evaluated 78 hours of video material from wild hummingbirds. It was found that birds with colorful male plumage were more likely to be the ones that drove others from feeding places and hunted, while female-plumed hummingbirds were more than ten times as likely to be victims instead of aggressors in such hunts. On the other hand, the researchers marked around 150 hummingbirds with RFID chips and set up several feeding stations in their habitat in Gamboa, Panama, which, using the chips, registered when one of the marked birds landed there and how long it stayed at the feeding station.
“Our tests showed that the typical, less colorful females were harassed much more than females with male-looking plumage,” reports Falk. “Since the females with their male plumage were exposed to less aggression, they could eat more often and for longer.” This is an important advantage, especially with hummingbirds, which have a very high need for food due to their high metabolic rate.
What use are typical female colors?
But why, despite this advantage, do the majority of adult females wear camouflage colors? The researchers do not have a clear answer to this. Although the typically female colored females are preferred when choosing a partner, the male-looking females also find mating partners. “In this case, the male coloring does not limit the evolutionary fitness of the females,” the researchers said. “One possible explanation, however, could be that the better camouflaged females are less likely to be victims of predators during breeding.”
It is still unclear which genetic mechanisms underlie the different coloring or whether environmental influences play a role. In future studies, Falk and his colleagues also want to investigate how similar variations affect other species.
Source: Jay Falk (Cornell University, Ithaca, New York) et al., Current Biology, doi: 10.1016 / j.cub.2021.07.043