Diving behavior of blue-footed boobies Sula nebouxii in northern Peru in relation to sex, body size and prey type

We studied the diving behavior and diet of the sexually size-dimorphic blue-footed booby Sula nebouxii (BFB) breeding on Isla Lobos de Tierra, Peru, to evaluate whether sexual dimorphism is associated with feeding niche divergence between the sexes. We also discuss the foraging behavior of BFBs in relation to what is known about the vertical distribution of their main prey, and compare the BFB's foraging behavior with that of other tropical and temperate Sulidae. After analyzing data on 23 females and 28 males, we found no sexual differences in diet composition, crop mass, trip length, number of trips per day, proportion of time in flight and on the water, and number of dives per hour. We argue that this is because males and females were foraging in the same areas close to their colony. Conversely, females dived significantly deeper and longer, and consumed larger prey than males indicating that segregation may occur underwater. Body size (regardless of sex) was positively correlated to dive depth and prey size, suggesting that feeding niche separation is mediated by size dimorphism. However, the effects of body size and sex on foraging behavior could not be separated. BFBs and larger temperate gannets attained similar depths, but BFBs dived much deeper than did tropical boobies. The Peruvian anchovy Engraulis ringens was the most important prey consumed by BFBs, accounting over 79 to 97 % of the total mass of the regurgitations. The temporal distribution of dives and dive profiles seems to be linked to the behavior and vertical distribution of anchovies.