His heart takes off like a trapped bird

[shootingstar]

(William, the chorister, is getting the stick)

Now he's actually standing there, William wishes he'd asked Martin (his friend), who is waiting his turn outside the study, to tell him exactly what was going to happen.
Mr. Atkinson stands next to William, and his heart takes off like a trapped bird. What now? Does he bend down? Does he wait to be told to bend down? How far does he bend? All the way? Halfway? Why the hell didn't he ask Martin?
'Bend over.' . . .

(From A Terrible Kindness by Jo Browning Wroe, Part II Cambridge Choir, chapter 30)

Please, what does 'take off' mean there?

 

[teechar]

It means leaps, or kicks into action. It's metaphoric.
In other words, his heart starts beating faster.

 

[shootingstar]

Thank you very much. I still have an issue with 'a trapped bird'. In my opinion the trapped bird doesn't take off but its heart does. So, I think a preposition is missing there and it has to be '. . . his heart takes off like (that) of a trapped bird'. Maybe this is added unconsciously or in thought.

 

[teechar]

No. Imagine a bird that has been trapped and then is suddenly allowed to fly. What would it do?

 

[shootingstar]

Do you think of the hasty and anxious fluttering of the trapped bird?

 

[teechar]

Right. I imagine the bird would seize the opportunity and fly off quickly.

 

[Flask]

. . . it has to be '. . . his heart takes off like (that) of a trapped bird'. Maybe this is added unconsciously or in thought.

If something "is added unconsciously or in thought" here, it is additional information about the trapped bird:

His heart takes off like a trapped bird [that has just been freed].