SWAMPS AND SHORES 



487 



cedure is possible because of the reduced food requirement of the trop- 

 ical climate and the time for the development of the young is not so 

 sharply limited. 



The guano production on tropical islands has been mentioned above 

 (p. 171). Wherever rainfall is slight or absent, as on the islands off the 

 coast of Peru, the bird excrement accumulates to depths of 7 to 30 m. 

 Dried by the tropical sun, the guano retains its nitrogen, which is not 



Fig. 127. — Breeding place of Sterna juliginosa on Ascension. All the birds 

 sit with the bill against the wind, to avoid ruffling of their feathers. After 

 v.Drygalski. 



given off as NH 3 . More than ten million tons of high-grade guano were 

 shipped from these islands between 1851 and 1872. The principal guano 

 producers are the cormorant Phalacrocorax bougainvillii, a pelican 

 (Pelecanus thagus) , and next to these the boobies {Sula spp.). On the 

 south island of the Chincha group a cormorant colony covers 60,000 

 sq. m., with three nests to the square meter, so that the assemblage 

 includes 360,000 adult birds. Shear-waters {Puffinus) and dwarf pen- 

 guins (Spheniscus demersus humboldti) breed in holes excavated in 

 the guano. Lams dominicanus and two vultures* are the principal nest 

 robbers. 28 



In coral islands used as breeding sites by oceanic birds situated in 



* Carthartes aura and Coragyps foetcns. 



