Sharper, straighter, stiffer, stronger: Male green hermit hummingbirds have bills evolved for fighting

(Press-News.org) Let’s get one thing out of the way: All hummingbirds fight. Most species fight for food, using their tiny bodies and sharp bills to force competitors away from flowers. But the green hermit hummingbird, which lives primarily in mountain forests of Central and South America, fights to win a mate.

“They gather together at a place in the forest that looks just like a singles bar,” said Alejandro Rico-Guevara, an associate professor of biology at the University of Washington. “They all have perches, and if someone else takes their perch — their place in the singles bar — they go bananas, and they fight.”

Hummingbirds’ weapon of choice? Their own bills. Like medieval knights in a joust, the birds raise a long, needle-thin bill into the air before driving it into their opponent. The stakes are high: Hummingbirds also use their bills to eat, poking them deep into flowers to reach nutrient-rich nectar. Losing a fight means a hummingbird might not find a mate. Breaking a bill could mean they starve. 

New research led by researchers at the UW Burke Museum of Natural History and Culture, where Rico-Guevara is the curator of birds, found that these fights have shaped the species’ evolution, yielding significant differences in bill shape for male and female green hermits. Compared to their female counterparts, male green hermits’ bills are straighter, sharper and structurally stronger. The straighter bills work better as weapons, while female birds’ more curved bills provide improved access to nectar in some flowers. The findings suggest that green hermits’ bill sexual dimorphism — when two sexes of a species exhibit different characteristics — was likely driven by their tendency to fight, not solely by ecological factors.

“Adult male green hermits have reinforced bills because they fight so much,” Rico-Guevara said. “It’s the same tool, but in very different contexts. This is an example of how much we can still learn from sexual dimorphism in nature.” 

In the study, published Nov. 10 in the Journal of Experimental Biology, researchers selected green hermit specimens from the Ornithology Collection at the Burke Museum and used photogrammetry to develop 3D models of male and female bills. Through curvature and angle analyses of those models, researchers found that male bills are 3% straighter and 69% sharper, respectively, with a dagger-like tip not found on female bills.

But the differences, researchers found, extend beyond bill shape. CT scans revealed that the male bill’s internal structure provided additional strength by transmitting forces more efficiently. 

Finally, researchers ran the models through a series of simulated stabbings to stress-test the bills in both head-on and angled attacks. They observed that the male’s straighter bill expends 52.4% less energy due to deformation, and is more resistant to breaking. The male bill experienced on average 39% less stress than the female bill. 

They also found that male bills’ straighter shape can accommodate a wider variety of attack angles, requiring less precision while fighting. 

“It’s a really cool example of sexually dimorphic weapons in birds,” said co-author Lucas Mansfield, a graduate student at Michigan State University who completed this work while studying at the UW. “When you think of sexually dimorphic weapons, you usually think of deer and moose, animals with big antlers. There aren’t many examples of things like that in the bird world. It’s fun to explore a more cryptic or hidden weapon.”

Co-authors include Felipe Garzón-Agudelo of the Centro de Investigación Colibrı́ Gorriazul in Colombia and Kevin Epperly, Ornithology Collections Manager at the UW Burke Museum. This research was funded by a Walt Halperin Endowed Professorship at the UW Department of Biology and by the Washington Research Foundation. 

For more information or to contact the researchers, email Alden Woods at acwoods@uw.edu. 

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