Kelenken

Kelenken is an extinct genus of giant flightless predatory birds of the family Phorusrhacidae or "terror birds".

K. guillermoi lived in the Middle Miocene, some 15 million years ago, in Argentina along with Argentavis. With a skull 28 inches (71 cm) long (including its 18 inch, 45.7 cm beak), it had the largest head of any known bird. It is the largest species of phorusrhacid. The tarsometatarsus was about 45 cm long

Diet
It is not entirely clear how Kelenken captured and killed its prey. As a large flightless carnivore, Kelenken likely chased down and killed its prey with several bone-shattering blows from its massive beak. Another possibility is that it may have picked up its prey item, then proceeded to shake it vigorously in order break its back. It is also possible that Kelenken may have been a scavenger, driving off other predators from their kills with its impressive size.