Weighing in at just 0.2 ounces, the Long-billed Hermit doesn’t seem like it would pose much of a threat to anything. But hummingbirds have what biologist Alejandro Rico-Guevara calls an “extremely pugnacious nature.” The cute little birds are feisty fighters, and when the normally solitary males come together to attract females with their singing and then compete for a chance to mate, scuffles often break out. One male will approach another and try to chase him away from his singing perch, and then the two will dance around and attempt to mount each other while bobbing their heads and snapping their beaks. As the confrontation escalates, Rico-Guevara has seen the birds attempt to stab each other in the throat with their beaks, using them like little daggers. 

It’s these beaks that set male and female Hermits apart. They look very similar to one another, except that the female and juvenile birds have even top and bottom beaks, while adult males have a distinct overbite and the top part of the beak ends in a sharp, needle-like point. In other hummingbird species where where males’ and females’ beaks aren’t the same, the difference is usually explained by feeding habits. The two sexes feed on different plants to limit competition for food, and their beaks are adapted to their different menus. But male and female Long-billed Hermits all feed from the same plants, and after watching the males stab at each other, Rico-Guevara wondered if the males’ pointier bills had evolved for fighting instead of feeding. 

To find out, he and biologist Marcelo Araya-Salas studied the birds during four breeding seasons in Costa Rica. They compared the size and shape of male and female beaks, tracked the development of the needle-like tips in males as they grew, tested the puncturing capability of the different birds’ beaks and watched as the males fought over mates. 

The researchers confirmed that only male birds had the longer, pointier upper beaks, and that they developed them only as they transitioned into adulthood and were ready to start mating and competing with each other. The adult males’ beaks were also able to perforate a plastic sheet more easily than the females’ or the young birds’ beaks and, in real-world fights, the birds with the largest and pointiest beaks were indeed better able to defend their territory and gain more access to females for mating. All of this, er, points to the sharp-tipped beak evolving as a weapon for male-vs.-male combat, says Rico-Guevara, and makes it the “the first documented sexually dimorphic weapon in hummingbirds.”

You can see the the hummingbirds fight and land a few blows with their face swords in this video from the researchers.