Impetuosity - a trait of mammals and birds (4,11)

'impetuosity' is the definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

'a trait of mammals and birds' is the wordplay.
I cannot really see how this works, but
'a' is within the answer.
'trait' could be 'blood' (blood is a kind of trait) and 'blood' is found within the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?