A colony of Humboldt penguins Spheniscus humboldti in central Chile was monitored from August 1995 to July 2000 to determine patterns of breeding and colony attendance and how these were affected by climatic (rainfall) and oceanographic (El Niño) factors. Nests were periodically checked for contents and roosting birds were counted from vantage points. Two main breeding events were observed: between August and January (spring event) and between April and June (autumn event). Whereas the spring event regularly produced offspring, the autumn event was systematically affected by rains, causing considerable nest desertion. Adults were present in the colony from August to May, abandoning the colony during winter after the nests were flooded. Juveniles occurred only between November and March. Adults moulted mainly in February, while juveniles moulted in January. During the 1997/98 El Niño episode, the number of breeding pairs was 55 to 85% lower than the mean, the onset of nesting was delayed, and abnormally heavy rainfall flooded nests. While the number of breeding pairs was significantly related to sea surface temperature anomalies (SSTA), breeding success was not. The attendance of adults and juveniles at the colony during El Niño was 25 and 73% lower, respectively, than the mean attendance. This 2-peak breeding strategy of Humboldt penguins appears to have evolved in response to the more favourable oceanographic and climatic conditions of Perú, where breeding is continuous and not interrupted by rains. Although less productive, the species probably maintains its autumnal breeding in central Chile because this provides additional offspring to supplement those regularly produced during the spring event.KEY WORDS: Humboldt penguin · Spheniscus humboldti · El Niño · Nest desertion · Breeding · Colony attendance Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 227: [43][44][45][46][47][48][49][50] 2002 with the tropics where little, if any, seasonal environmental fluctuation occurs and bird breeding may extend over a longer period (Lack 1954, 1967, Furness & Monaghan 1987. Nearly all marine birds follow this synchrous breeding strategy (Lack 1967).The Humboldt penguin Spheniscus humboldti breeds over 4500 km along the Pacific coast of Perú and Chile between 5°and 42°S (Williams 1995). Over this extensive distribution area the species faces considerable latitudinal gradients in oceanographic and climatic factors, including marine productivity, El Niño events, sea surface temperatures and rainfall (see Hunt & Schneider 1987). This situation is likely to expose the species to differential selective pressures over its breeding range.In their northernmost breeding areas along the subtropical Peruvian coast, Humboldt penguins nest throughout the year with well-defined breeding events in autumn and spring (Murphy 1936, Zavalaga & Paredes 1997. Whereas adults remain permanently near the colonies, juveniles are absent for several months after fledging (Zavalaga & Pared...

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