Seabird guano consists of nitrogen-rich
ammonium oxalate and
phosphates, as well as some earth salts and impurities. Unleached guano from favored locales, such as the
Chincha Islands off the coast of
Peru, typically contains 8 to 16 percent nitrogen (the majority of which is
uric acid), 8 to 12 percent equivalent phosphoric acid, and 2 to 3 percent equivalent
 The fresh excrement of insect-eating bats can have nitrogen levels approximating that of seabird guano and high phosphate levels, but bat guano typically has less fertilizing value than seabird guano from arid areas because its nitrogen content is more easily leached within cave environments.