The naturally desiccated, nitrogenous dung of fish-eating sea fowl which is one of the best organic fertilisers. Guano can only be deposited (and preserved) in great quantities if certain climatic and geographical factors are met. Aridity is essential, as are also abundant food supply of fish and sufficiently large nesting grounds to favour a large concentration of sea-birds. Especially rocky islands on the continental shelf of Peru and Chili fulfil these requirements.