It started with a mystery: why are so few bird species armed and dangerous?
‘Weapons’ are commonplace in the animal kingdom – from horns and antlers to sharp teeth and claws, many species have specialised anatomical structures for fighting. But birds appear to be an exception.
“It’s kind of puzzling,” says João CT Menezes, a graduate student at the University of Massachussetts Amherst, US, and co-author on a new paper published in Ecology Letters.
“Birds have such spectacular songs, plumage and dances, but they mostly don’t have specialised weapons. It’s strange because dancing, singing, fancy feathers and fighting are all ways of successfully obtaining a mate, and often go together.”
It turns out that the evolution of flight might have something to do with birds’ lack of fight.
Together with Alexandre V Palaoro, a postdoctoral fellow at Clemson University in South Carolina, US, Menezes started gathering data on weapons and flight across bird species.
Them’s fighting birds
A select few bird species – 1.7%, according to Menezes and Palaoro’s dataset – do have bony spurs on their legs or wings that act as weapons.
“Spurs are typically used in fights for mates or territory, but they can also be used against predators,” says Menezes.
While spurs are comparatively rare in the bird world, chickens and turkeys are among the familiar species that have them. (In fact, it’s thought that chickens were originally domesticated and raised for cockfighting, and only began to be raised as food a little over 2000 years ago in Europe and the Middle East.)
But just how does a bird wield its weapons, you might be wondering.
“There are basically two fighting strategies that correlate to the location of the spurs,” Menezes says.
“Birds with wing spurs – lapwings, jacanas, sheathbills, and some species of ducks, geese, and doves – use them to hit opponents with pounding movements. Steamer ducks, for example, grasp their opponent by the head and beat them with their flashy orange knobs.
“Birds with leg spurs – junglefowl, turkeys, pheasants, and many of their relatives – use them much like roosters: with jump kicks.”
Correlating fight with flight
The researchers compared the presence of spurs to the hand-wing index (HWI), which summarises birds’ efficiency when taking flight.
“Low HWI denotes short, rounded wings appropriate for short or occasional flights, while high HWI is found in narrow, pointed wings – good for long or frequent flights,” Menezes explains.
He and Palaoro discovered an interesting correlation between the two datasets.
“The best fliers tend to lack spurs, and the most heavily armed fighters tend to struggle in the air,” says Menezes.
Simulations suggested that spurs make flight more energy-intensive, potentially explaining the relationship. Larger spurs might help a bird dominate its rivals, but they could make it worse at flying away from predators, or create a need to eat more food to get enough energy. Over time, natural selection may have favoured the spur-less option for most species.
“Flight costs more energy in the form of burned fat the heavier you are,” Menezes says. “This is why airlines are always trying to reduce weight on flights to save fuel, for example.”
Flying birds have evolved adaptations like hollow bones and lack of teeth to improve their flight, he points out.
“Our results potentially put lack of specialised weapons right next to these textbook examples that are often taught in secondary school.”
The evolution of diverse plumage, song and dancing behaviour may have allowed birds to resolve conflicts over mates and territory without physical fights, he suggests.
Shedding light on evolution
There’s still another wrinkle: how do flightless birds, like emus or penguins, fit into this evolutionary story?
“This is an excellent question,” Menezes says. “While we have found that birds that fly more have on average fewer or no spurs, species that are altogether flightless – like ratites, penguins, and kiwis – typically lack these weapons, which is very curious.”
He offers a couple of ideas for why this could be.
“One possible explanation for why these groups lack spurs is simply that their ancestors also lacked them.
“A complementary reason would be that for some reason weapons don’t offer too many benefits to individuals in these groups — for example, their ecology might be so that they don’t need to defend territory or mates.”
In future work, Menezes would like to investigate whether fight or flight came first, evolutionarily speaking. That is, did the ancestors of birds have spurs that were lost in the evolution of efficient flight, or did spurs develop after flight only in the birds who needed them?
“The most robust piece of evidence would be if spurred fossils were found in early bird fossils,” he says. “We’re aware of one fossil from around that time that was spurred, but Archaeopteryx, for example, which is typically regarded as the first modern bird, lacked spurs.”
Another future step would be to see whether the negative correlation between flight and weapons exists in other animals.
“Morphological weapons are apparently also rare (or maybe entirely absent) in bats and in insect groups that are good fliers, like butterflies, dragonflies, and flies,” says Menezes.