Flight hampers the evolution of weapons in birds.

Birds are a remarkable example of how sexual selection can produce diverse ornaments and behaviours. Specialised fighting structures like deer's antlers, in contrast, are mostly absent among birds. Here, we investigated if the birds' costly mode of locomotion-powered flight-helps explain the scarcity of weapons among members of this clade. Our simulations of flight energetics predicted that the cost of bony spurs-a specialised avian weapon-should increase with time spent flying. Bayesian phylogenetic comparative analyses using a global spur dataset corroborated this prediction. First, extant species with flight-efficient wings (which presumably fly more frequently) tend to have fewer or no bony spurs. Second, this association likely arose because flying more leads to more frequent evolutionary loss of spurs. Together, these findings suggest that, much like pneumatic bones, absence of weaponry may be another feature of the avian body plan that allows birds to efficiently explore the aerial habitat.