Abstract
In fl uctuating environments, matching breeding timing to periods of highresource availability is crucial for the fi tness of many vertebrate species, and may havemajor consequences on population health. Yet, our understanding of the proximate environmentalcues driving seasonal breeding is limited. This is particularly the case in marineecosystems, where key environmental factors and prey abundance and availability areseldom quantifi ed. The Northern Humboldt Current System ( NHCS ) is a highly productive,low- latitude ecosystem of moderate seasonality. In this ecosystem, three tropical seabirdspecies (the Guanay Cormorant Phalacrocorax bougainvillii , the Peruvian Booby Sula variegata ,and the Peruvian Pelican Pelecanus thagus ) live in sympatry and prey almost exclusivelyon anchovy, Engraulis ringens . From January 2003 to December 2012, we monitored 31breeding sites along the Peruvian coast to investigate the breeding cycle of these species.We tested for relationships between breeding timing, oceanographic conditions, and preyavailability using occupancy models. We found that all three seabird species exhibitedseasonal breeding patterns, with marked interspecifi c differences. Whereas breeding mainlystarted during the austral winter/early spring and ended in summer/early fall, this patternwas stronger in boobies and pelicans than in cormorants. Breeding onset mainly occurredwhen upwelling was intense but ecosystem productivity was below its annual maxima, andwhen anchovy were less available and in poor physiological condition. Conversely, theabundance and availability of anchovy improved during chick rearing and peaked aroundthe time of fl edging. These results suggest that breeding timing is adjusted so that fl edgingmay occur under optimal environmental conditions, rather than being constrained by nutritionalrequirements during egg laying. Adjusting breeding time so that fl edglings meetoptimal conditions at independence is unique compared with other upwelling ecosystemsand could be explained by the relatively high abundances of anchovy occurring throughoutthe year in the NHCS .