Poor, dead, ugly bird, corpse nibbled by crabs and devoured by fish. For a stunning description of this peculiar, prehensile bird, see Adam Nicolson, in his marvelous Sea Room.  Elsewhere Williamson speaks of ‘crab-green’.

‘Feeding with other fish on the crab-nibbled corpse of the shag.’

See also a quote-rich mosaic review of this wonderful classic. 

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. 95