‘The owl was hearkening, however, for the prick of the claws of mice on leaves, and when it heard these tiny noises, it stared until it saw movement, and with a skirling screech that made the mouse crouch in a fixity of terror sailed to the ground and clutched it in a foot.’

A Scottish and northern English dialect word meaning to screech, shriek or otherwise make a shrill noise. By extension it’s used to describe the sound or music of a bagpipe, or as a verb for playing the bagpipe.

OED gives an additional meaning of ‘flying with a sweeping or whirling motion’, so the owl that struck the cowering mouse could have skirled both in sound and flight.

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 15