whistler vs thickhead

whistler and thickhead both are nouns.

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

As nouns, whistler and thickhead are synonyms defined as:

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

whistler (noun)	thickhead (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