Sharper, straighter, stiffer, stronger: sexually dimorphic bill shape enhances male stabbing performance in the green hermit hummingbird ( <i>Phaethornis guy</i> )

Bill sexual dimorphism has been primarily linked to differential use of food resources between sexes. However, intrasexual selection has been suggested to also influence hummingbird bill morphology. Males of Phaethornis longirostris (long-billed hermit) possess sharp, elongated, dagger-like bill-tips that enhance puncturing ability and territory defense during male-male lekking combat. Yet, the prevalence of weaponized bills in hermits and the impact of bill shape on fighting performance remain unexplored. We employed 3D modelling and finite element analysis to explore bill dimorphism and stabbing performance in another lekking hummingbird: Phaethornis guy (green hermit). Our results reveal that P. guy exhibits a male-specific bill-tip dagger, and that males' straighter bills show greater biomechanical performance during stabbing tests, transmitting forces more efficiently (strain energy) and reducing the risk of breakage (von Mises stress). These findings provide further evidence of bill-tip weaponry and support a role of agonistic interactions in the evolution of bill dimorphism.