Apteryx and Flightless bird

Apteryx noun - Nocturnal flightless bird of New Zealand having a long neck and stout legs; only surviving representative of the order Apterygiformes.

Flightless bird and apteryx are semantically related. In some cases you can use "Flightless bird" instead a noun "Apteryx".

Flightless bird noun - Flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches; cassowaries; emus; moas; rheas; kiwis; elephant birds.

Apteryx and flightless bird are semantically related. Sometimes you can replace term "Apteryx" with "Flightless bird", this terms are similar.