A symbiosis of hovering birds and fly-pestered bullocks, the darting martins hoovering up bothersome crisp-winged flies.

‘Martins twittered along the river-bank, and hovered about the heads of bullocks, taking crisp-winged flies from their muzzles and between their horns.’

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. 142