From May 1996 to June 1998, we studied the feeding ecology of Peruvian (Sula variegata), Blue-footed (S. nebouxii) and Masked boobies (S. dactylatra) on Lobos de Tierra Island, at the northern edge of the Peruvian Upwelling System. During this period, changing oceanographic conditions affected prey availability: 1996 was cold and 1997-1998 was an El Nino event. The three species showed different responses to changes in the surrounding marine environment. Peruvian Boobies fed exclusively on Peruvian anchovies (Engraulis ringens) during both 1996 and 1997 and were not present on the island in 1998. Blue-footed and Masked boobies fed extensively on this prey during 1996, less so in 1997, and no anchovies were found in their diet in 1998. In 1997-1998, Blue-footed Boobies switched to coastal fishes other than anchovies, and Masked Boobies fed almost exclusively on oceanic prey species. Changes in regurgitate mass and mean number of prey items in regurgitates were also observed. Results suggest that the abundance and availability of fish prey species, mainly Peruvian anchovies, determines thresholds at which booby species change foraging strategies to avoid possible competition for less available resources. Received 13 June 1999, accepted 28 October 1999.