[romillyh]

Can't help too much as I use equipment that you wouldn't want to buy just to do this! You may be able to do something with a portable music player that has an internal mic, though often the quality is poor (noisy recording masking the sound you want, specially if far away). Then your options are to sit at the window waiting to turn on when you hear the bird, leave the recorder running in the window while you sleep, or go out to where the bird usually calls and wait there with a finger on the button!

You'd need some way of getting the recordings onto a computer and some sound editing software to make mp3 or wav clips to post.

Simpler alternative may be to download some barn owl clips and play them until the sound's in your head and listen again. The wheezing barny screech is very distinctive.