signaller vs whistler

signaller and whistler both are nouns.

Word	Noun	Adjective	Verb	Adverb
signaller	Yes	No	No	No
whistler	Yes	No	No	No

As nouns, whistler is a hyponym of signaller; that is, whistler is a word with a more specific, narrower meaning than signaller:

Other hyponyms of signaller include signalman, bell ringer, ringer, toller.

signaller (noun)	whistler (noun)
someone who communicates by signals	Australian and southeastern Asian birds with a melodious whistling call
	large-headed swift-flying diving duck of Arctic regions
	large North American mountain marmot
	someone who makes a loud high sound