Could this be translated as "flightless birds"? Or is there a separate word/phrase for them?
If we take the literal meaning of "flightless birds", then yes it has the same meaning. However, "flightless birds" has a special meaning in English referring to species of birds where the whole species is unable to fly. The Klingon can be used to describe such birds, but is not limited to them and can also refer to birds with broken or clipped wings, for instance. There is no short phrase that I know of to refer to birds whose species is flightless and it would take a longer, probably multi-sentence description to get that meaning across.