Hummingbirds, usually considered sweet, harmless sippers of nectar, aren't so sweet when it come to competing for mates, using their beaks to "go for the throat" of rivals, researchers have found.

Although it has generally been assumed the hummingbirds evolved their long, slender beaks in order to collect nectar deep within flowers, it turns out they may have also developed their distinctive form to be useful as weapons.

At least that appears to be the case in the males of long-billed hermit hummingbirds, natives of tropical Costa Rica, which utilize their beaks as weapons to try to stab each other in the throat mating contests, the researchers report in the journal Behavioral Ecology.

The males, upon reaching the age when they start looking for mates, grow sharp dagger points on their already long bills, they say.

"Historically, bird beaks have been the prime example of adaptation through natural selection, such as in the textbook example of Darwin's finches," says Alejandro Rico-Guevara, University of Connecticut researcher in evolutionary biology.

"But we show here the first evidence that bills are also being shaped by sexual selection through male-male combat," he says. "It is exciting to think of all these forces working on the way animals look, and to think about how they might affect males and females differently."

Males and female hermit hummingbirds, Phaethornis longirostris, have beaks with differently shapes, something previously put down to that fact they were seen to feed on different types of flowers.

But the researchers, witnessing the males fighting in their attempts to win mates, wondered if their beak shapes gave them an advantage when it came to their aerial combats.

To determine that, they compared the size and the puncture capability of beak tips in juvenile as opposed to adult hermit hummingbirds.

During the transition into adulthood, males developed longer beak tips that were sharper than those of females, they discovered.

Adult males were observed attempting to stab one another in the throat in mating contests, which generally were settled in favor of any males who possessed longer, pointer bills than their rivals.

Other hummingbirds species also have beaks seemingly suitable for use as weapons, Rico-Guevara says, and he plans more studies focusing on hummingbird evolution.

Despite their small size, he says, hummingbirds are proving themselves to be strong and quite aggressive creatures.

"I think people initially think of them as beautiful, delicate creatures," he says, "but I enjoy revealing their pugnacious attitudes."