New research finds that male green hermit hummingbird bills have evolved traits that suit both feeding and fighting. The green hermit lives primarily in mountain forests of Central and South America. Males gather at display sites and contest perches to secure mates, and during aggressive encounters they raise their long, needle-thin bills and drive them into opponents.
The study, led by researchers at the UW Burke Museum of Natural History and Culture with Alejandro Rico-Guevara as a lead author, used specimens from the museum’s Ornithology Collection. Scientists applied photogrammetry to build 3D models of male and female bills and used CT scans to examine internal structure. Analyses showed males have bills that are 3% straighter and 69% sharper, often with a dagger-like tip, and an internal form that transmits forces more efficiently during impacts.
Simulated stabbing tests on the 3D models found the male bill expended 52.4% less energy from deformation and experienced on average 39% less stress than the female bill. The straighter shape also allowed attacks from a wider range of angles, so males need less precision when striking. The authors interpret these results as evidence that fighting helped drive sexual dimorphism in green hermit bills; female bills remain more curved, which can improve access to nectar in some flowers. The study appears in the Journal of Experimental Biology.
- Coauthors: Lucas Mansfield, Felipe Garzón-Agudelo, Kevin Epperly
- Funding: Walt Halperin Endowed Professorship (UW biology) and Washington Research Foundation
Difficult words
- evolve — change over time by natural processeshave evolved
- specimen — an individual sample used for studyspecimens
- photogrammetry — method to make measurements from photographs
- deformation — change in shape due to force
- sexual dimorphism — physical difference between males and females
- precision — exactness or accuracy of an action
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Discussion questions
- Why might fighting drive differences in male and female bill shape in these hummingbirds? Give reasons based on the article.
- What are the possible benefits and costs for a male to have a straighter, sharper bill compared with a more curved bill?
- How convincing do you find the use of museum specimens and 3D models to study bill function and fighting? Explain your view.
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