seabird vs sea bird

seabird and sea bird both are nouns.

Word	Noun	Adjective	Verb	Adverb
seabird	Yes	No	No	No
sea bird	Yes	No	No	No

As nouns, seabird and sea bird are synonyms defined as:

seabird and sea bird: a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.

Other synonyms of seabird include seafowl.

seabird (noun)	sea bird (noun)
a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.	a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.