Guano is the accumulated excrement of seabirds and bats. As a manure, guano is a highly effective fertilizer due to its exceptionally high content of nitrogen, phosphate, and potassium: key nutrients essential for plant growth. Guano was also, to a lesser extent, sought for the production of gunpowder and other explosive materials. The demand for guano in the 19th century spurred the human colonization of remote bird islands in many parts of the world, resulting in some of the first examples of United States colonialism and the expansion of the British Empire. However, the guano-mining process resulted in ecological degradation through the loss of millions of seabirds. This photograph, taken near Île-de-Bréhat in northwestern France, depicts a herring gull (Larus argentatus) excreting waste, which accumulates to form guano.Photograph credit: Nicolas Sanchez