sally H wrote:
I have just bought vaious bird boxes and should like to put them up
around the garden.

The question is - how do you secure them to the tree trunks because
I
thought that banging a nail into the tree will kill or hurt it.

thanks

Tie them on.
When you clean the boxes out (you will, won't you?) you can adjust,
re-fit the ties.
But should you forget, as the tree trunk expands it will grow round
the tie with - IME - little damage to the tree.
Place them with the entrance facing roughly South-east, not close
together and away from paths and high traffic areas (seed feeders,
etc.) - they do appreciate some privacy.

--
ned