whistler vs true flycatcher

whistler and true flycatcher both are nouns.

Word	Noun	Adjective	Verb	Adverb
whistler	Yes	No	No	No
true flycatcher	Yes	No	No	No

As nouns, true flycatcher is a hypernym of whistler; that is, true flycatcher is a word with a broader meaning than whistler:

whistler: Australian and southeastern Asian birds with a melodious whistling call
true flycatcher: any of a large group of small songbirds that feed on insects taken on the wing

Other hypernyms of whistler include flycatcher, Old World flycatcher.

whistler (noun)	true flycatcher (noun)
Australian and southeastern Asian birds with a melodious whistling call	any of a large group of small songbirds that feed on insects taken on the wing
large-headed swift-flying diving duck of Arctic regions
large North American mountain marmot
someone who makes a loud high sound