EL NIÑO AND THE BIRDS: A RESOURCE-BASED INTERPRETATION OF CLIMATIC FORCING IN THE SOUTHEASTERN PACIFIC

Abstract

Through a review, we attempt to identify what aspects of El Niño-Southern Oscillation (ENSO) determine the availability of resources that affect avian ecology in the southeastern Pacific. We recognized three functional bird types: seabirds, waterbirds, and landbirds (the latter separately treated as granivorous, insectivorous, carnivorous). The evidence shows that El Niño affects seabirds by reducing their food base. Seabirds feeding on fish, or fish plus squid, generally decrease their population abundance, as a combined result of adult mortality and reproductive failure. Omnivorous seabirds negotiate El Niño periods with modest declines, while those including fishery wastes in their diets show minimal changes during such periods. Waterbirds in the southeastern Pacific benefit from precipitation that accompanies El Niño, which enlarges their wetland habitats. Finally, landbirds are favored by El Niño-driven precipitation, which fires up primary productivity fuelling arthropod and mammal increases. Granivorous birds react promptly to peaks in seed output, while insectivores respond similarly to arthropod abundance. Carnivorous birds take somewhat longer to react to increases in mammal populations. We conclude that (a) Food shortage may be an important key factor for bird populations in the southeastern Pacific, but its effects depend on diet breadth, prey switching, and life history parameters. (b) Seasonal patterns of productivity determine distribution and abundance of resident birds, but variations imposed by occurrences of El Niño and La Niña phases determine the presence and abundance of migratory birds. (c) The dichotomy El Niño versus La Niña or versus 'normal' years is artificial and non conducive. (d) Long-term monitoring is needed to understand the connection between climate and birds.