It depends. If it's plural: robins'.
If it's singular: robin's.
The possessive form of the word "dictionary" is "dictionary's."
The singular possessive form is word's, for example the word's definition.
possessive form of the word glass: glass'
possessive form of the word Congress: Congress'
possessive form of the word Cathi: Cathi's
possessive form for the word holders: holders'
The possessive form of the word fairy is fairy's.
the possessive form of the word Cathie: Cathie's
The possessive form is Lindbergh's.
The possessive form is weaver's.
The possessive form for the interrogative pronoun who is whose.
The singular possessive form of the word "jury" is "jury's."