Bird beset by pests died covered in growths? (8)

'covered in growths?' is the definition.
The definition suggests a past participle verb which matches the answer.

'bird beset by pests died' is the wordplay.
'bird' becomes 'hen' (hen is a kind of bird).
'beset by' means one lot of letters goes inside another ('beset' could historically mean 'cover').
'pests' becomes 'lice' (lice are pests).
'died' becomes 'd' (abbreviation next to the year of someone's death).
'hen' placed into 'lice' is 'lichene'.
'lichene'+'d'='LICHENED'

Can you help me to learn more?