singer vs thrush

singer and thrush both are nouns.

Word	Noun	Adjective	Verb	Adverb
singer	Yes	No	No	No
thrush	Yes	No	No	No

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

singer (noun)	thrush (noun)
a person who sings	songbirds characteristically having brownish upper plumage with a spotted breast
	a woman who sings popular songs
	candidiasis of the oral cavity; seen mostly in infants or debilitated adults