Submitted by violetmammal4694 t3_1242piw in askscience
I ask because I know that most extant birds (either flying or flightless) are omnivores.
I also know that the only two extant birds species that are obligate herbivores are the unique hoatzin and the strange kakapo, one is a poor flyer, the other is really flightless, but a great climber.
Formerly, there were nonmarine carnivorous flightless birds such as phorusrhacids (or terror birds, if you want), and the mysterious Jamaican caracara (these two examples are both australavians like the kakapo, but the former is related to seriamas, and the latter was technically the world's only flightless species from the order Falconiformes). Since they are all dead, the only living carnivorous birds are the famous penguins, and the critically endangered flightless cormorant. Both penguins and cormorants are marine obligate carnivores.
However, I think there still could be an extant nonmarine carnivorous flightless bird: the kagu (this species is exclusively found in New-Caledonia, which is part of the Fifth French Republic).
andanother12345 t1_jdz76ep wrote
You've done your research.
The only additional extant species I can think of are more honorable mentions. Sagittariidae are mostly terrestrial, but capable of flight and nest in trees.
Ground cuckoos in the subfamily neomorphinae also aren't truly flightless, but are terrestrial. They aren't true carnivores, but they are fierce predators and will prey on rattlesnakes and tarantula hawks.