hobby vs pigeon keeping

hobby and pigeon keeping both are nouns.

Word	Noun	Adjective	Verb	Adverb
hobby	Yes	No	No	No
pigeon keeping	Yes	No	No	No

As nouns, pigeon keeping is a hyponym of hobby; that is, pigeon keeping is a word with a more specific, narrower meaning than hobby:

Other hyponyms of hobby include spelaeology, speleology.

hobby (noun)	pigeon keeping (noun)
an auxiliary activity	the art and science of breeding domestic pigeons.
small Old World falcon formerly trained and flown at small birds
a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride