Scientists have long known that the males and females of the long-billed hermit (Phaethornis longirostris), a large and long-billed hummingbird that can be found in Costa Rica, have differently-shaped beaks and this was previously attributed to their feeding habits.

Alejandro Rico-Guevara, from the Department of Ecology and Evolutionary Biology at the University of Connecticut, said that the earlier explanation on the dissimilarity between the beaks pointed at the female and male birds of the species feeding on different flowers.

Findings of a new research conducted by Rico-Guevara and Marcelo Araya-Salas, from the New Mexico State University, however, found that the beak of hummingbirds is not just for probing flowers for nectar. Male hummingbirds also use their long and sharp bill as a weapon for stabbing each other in the throat in battles over a mate.

Rico-Guevara observed that the adult males extensively used their beaks during fights and set out to conduct an investigation with Araya-Salas to find out more about the bird's beak. They discovered that as the male birds transition into adulthood, they develop elongated beak tips that were sharper when compared with those of the female.

Part of the bird's mating ritual involves leks, wherein male hummingbirds battle so they can have space to mate with the females.  The researchers observed that during fights, the males with longer and pointier beak had increased odds of winning in battles over territories.  

"Once a female is in a territory, the male will court her with elaborate displays and songs. So in these species the males are constantly fighting to maintain the best territories," Rico-Guevara said.

The researchers' findings also offered hints on the evolution of the bird's beaks. It is believed that the bills of hummingbirds evolved so the birds could easily access the nectar of the flowers that they frequently visit. The study suggests that it was the flowers that evolved to conform to the shape of the beaks that the males use when battling with other males.

"Our study provides the first evidence of sexually dimorphic weapons in bird bills and stands as one of the few examples of male weaponry in birds," the researchers wrote in their study, which was published in the journal Behavioral Ecology on Oct.18. "Our results suggest a role of sexual selection on the evolution of overall bill morphology, an alternative hypothesis to the prevailing "ecological causation" explanation for bill sexual dimorphism in hummingbirds."