Sculpture includes heads of Hoover, Truman, Ike and songbird?

'songbird?' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps you can see an association between them that I don't see?

'sculpture includes heads of hoover truman ike' is the wordplay.
'sculpture' becomes 'bust' (bust is a kind of sculpture).
'includes' means one lot of letters goes inside another.
'heads of' says to take the initial letters.
The initial letters of 'hoover truman ike' is 'hti'.
'bust' placed around 'hti' is 'BUSH-TIT'.

'and' is the link.

Can you help me to learn more?