Predatory feeding behavior in terror birds. The common name for the extinct flightless phorusrhacids is “terror bird,” an apt name indeed. Phorusrhacids radiated in South America starting about 60 million years ago, eventually migrating to North America before going extinct relatively recently. No one has ever doubted that terror birds were predators, but without modern-day counterparts, their lifestyles have been obscure. A new study looked at the 6-million-year-old skull of Andalgalornis , a patagornithine terror bird found in northwestern Argentina. CT scanning allowed a multinational team to probe the inner anatomical structure of its skull, as well as to perform advanced engineering analyses of its skull biomechanics. The anatomical studies showed that terror birds had also evolved very rigid heavy skulls. Bird skulls generally are lightly built and very flexible with lots of internal joints, but Andalgalornis had evolutionarily transformed all those flexible joints into strong rigid beams with relatively thick bone. Loss of cranial kinesis (internal flexibility) is rare in birds, and so in terror birds it must be linked to increasing the rigidity of the skull particularly in the fore-aft direction. An engineering method called finite-element analysis allowed different feeding behaviors for Andalgalornis to be simulated and evaluated. The simulations told the same story as the anatomy: the skull was well-adapted to drive the skull straight down, as well as to tug straight back, but couldn’t tolerate thrashing the head from side to side. The findings suggest that terror birds used their powerful necks to jab prey with their rigid bills like a boxer, using their speed and agility to repeatedly attack and retreat, but being careful not to grapple with the prey too much which might damage the tall narrow beak. Once killed by the axe-like beak, the prey would be swallowed whole or dismembered by grasping with the hooked beak and pulling straight back with the nec k.