Old birds back from tea in seconds (4)

'old birds' is the definition.
'moas' can be an answer for 'birds' (moa is a kind of bird). I am not certain of the 'old' bit.

'back from tea in seconds' is the wordplay.
'back from' indicates one should take the final letters (the letter at the back of the word).
'in' means one lot of letters goes inside another.
'seconds' becomes 'mos' (mo can mean a short amount of time).
The last letter of 'tea' is 'a'.
'a' put inside 'mos' is 'MOAS'.

Can you help me to learn more?