For years, the main driver of evolutionary change in hummingbird bills was thought to be feeding ecology, but a recent study by UC Berkeley researchers, published in the journal “Integrative Organismal Biology,” proves otherwise. Some male hummingbirds in the tropics of South America favor bills more suited to fighting than feeding.
According to lead scientist on the project and Miller Postdoctoral Fellow at UC Berkeley Alejandro Rico-Guevara, physical traits observed in male hummingbirds in the tropics of Central and South America could not be explained through adaptations to feeding strategies.
“They’re always jousting,” said Robert Dudley, co-author of the project and chair of the integrative biology department at UC Berkeley. “All those crazy hooks and things on the bills may make it harder to extract nectar.”
Straighter, pointier bills can transmit more force and act as stabbing weapons but simultaneously limit the male hummingbird’s ability to reach deep inside certain flowers, according to Dudley and his colleagues. They found that these male hummingbirds are “spending more energy fighting than feeding.”
The male hummingbirds possess stiff bills with pointed hook tips and rear-facing serrations — ideal for poking or snatching feathers from other hummingbirds. Rico-Guevara and his team filmed the hummingbirds with high-speed cameras and viewed interactions between two hummingbirds. They observed that the birds used their bills to bite skin or pluck feathers from each other.
One hypothesis from the study is that bill modifications better allow the male hummingbirds to hunt for arthropods, their primary sources of protein. But, Rico-Guevara said it is usually female hummingbirds that hunt for spiders and flies, as they require more protein. There were a series of other hypotheses that Rico-Guevara claimed “did not make sense” in regard to hummingbird morphology.
Natural selection for better foraging over feeding strategies has its trade-offs but benefits males in regard to mating, according to Rico-Guevara. Male hummingbirds are territorial creatures that seek to establish an area of flowers for both feeding and mating purposes. If male hummingbirds can ward off other males and claim a patch of flowers, they have all the resources to themselves, which compensates for their less-efficient feeding bills, according to Rico-Guevara.
“The males are the ones who have the weapons because they are the ones who fight amongst themselves,” Rico-Guevara said. “A weapon becomes a really good trait to have to establish this dominance.”
The change in bill structure has only been observed in the tropics because there is a greater diversity of flowers and hummingbirds, meaning more competition and more reason to have “weaponized” physical attributes, according to Rico-Guevara and Dudley. Tropical regions do not experience seasons as intensely as North America, so there is also a more stable supply of floral resources and opportunities to feed on different flowers.
This discovery can open doors into new ways to study the morphology and biology of hummingbirds, according to the researchers. Rico-Guevara said one of these doors may explore the birds’ flying abilities in relation to foraging and behavioral strategies.
“(This research is) changing the perspective and putting more emphasis on all aspects, especially on the aggressive side of their biology,” Rico-Guevara said.