thickhead vs whistler

thickhead and whistler both are nouns.

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

As nouns, thickhead and whistler are synonyms defined as:

thickhead and whistler: Australian and southeastern Asian birds with a melodious whistling call

thickhead (noun)	whistler (noun)
Australian and southeastern Asian birds with a melodious whistling call	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