We used miniature bird-borne data loggers to examine the foraging strategies of Blue-footed Boobies (Sula nebouxii) breeding on Isla Lobos de Tierra (06°24'S, 80°51'W). This colony is surrounded by the rich upwelling marine system along the Peruvian coast dominated by Peruvian anchovies (Engraulis ringens), booby's main prey.

The devices, designed and manufactured by the C.N.R. (Pisa, Italy), were attached on the bird's back and recorded for some days the main activities displayed during the breeding period (nest attendance, flight and resting on the sea surface, diving profiles, foraging routes). Trips of 20 boobies in winter 2002 were longer (mean 5,0 ± 3,5 h) and less frequent (1,1 trips per day) than those performed by 59 boobies in summer 2003 (2,8 ± 2,1 h; 1,7 trips per day). During the foraging trip birds mainly were flying (85%) and spent the rest of time on the sea surface (11%) or plunge-diving (4%). Diving occurred in the entire trip except the first and last period. Mean dive duration was 6,4 ± 4,4 s; dives in 2003 were deeper than 2002 (4,3 ± 1,9 m ; 3,6 ± 1,4 m). A small amount of dives were "U"-shaped, which indicates wing and/or foot underwater propulsion to gain additional depth. Females, larger and heavier than males, dove deeper and were capable of seizing larger preys. Foraging routes were predominantly oriented to the southeast, along the cold and rich waters of Peruvian coast, ranging 10 to 70 km from the colony (mean: 43 ± 27 km); in 2003 females foraged farther than males.

The seasonal differences exhibited rely on the Blue-footed Booby foraging strategy plasticity, well adapted to an environment with high availability but patchy distribution of food. Female higher diving ability and larger foraging range support the hypothesis of intersexual feeding-niche partitioning, associated with sexual size dimorphism.