The three species of guano birds, guanay cormorant (Phalacrocorax bougainvillii), Peruvian booby (Sula variegata), and Peruvian pelican (Pelecanus occidentalis thagus), depend mainly on the Peruvian anchovy (ingraulis ringens) for sustenance. A naturally occurring limitation on the population of these birds is the oceanographic phenomenon known at ''El Ni¿o''. The industrialization of the anchovy fishery during the 1960s and 1970s has also affected seabird populations: the annual extraction of huge volumes of fish has decreased the availability of food for the guano birds, causing large population fluctuations and considerable changes in the relative abundance of the birds. Because of its great intensity and duration, the 1982--1983 El Ni¿o caused high avian mortality, depressing populations to levels from which they have not yet recovered. ¿ American Geophysical Union 1987