The influence of El Niño on the spring fallout of Asian bird species at Attu Island

Several studies have documented the effect of the recent secular climate warming on the distributions and geographical ranges of birds. Here the authors report the strong impact of a recurring climatic pattern in the equatorial Pacific, the El Niño-Southern Oscillation (ENSO) cycle of warm (El Niño) and cold (La Niña) events, on spring migrants along the Far Eastern flyway in northeast Asia. In El Niño years, an unusually large number of birds that use the flyway are observed at Attu Island, westernmost of the Aleutian Islands, nearly 960 km away from the Asian coast. This study is based on a 20-yr dataset documenting the year-to-year variation of Asian birds arriving on Attu in the spring season and uses a three-phased analytical methodology to examine climate impacts on bird movements and populations. The authors offer evidence that birds are displaced toward the Attu area in strong eastward-moving storms. They also present results from a reverse trajectory model that was used to simulate trajectories that a sample of Attu arrivals likely followed in reaching the island. In a statistical analysis, it is shown that 79% of the variation of the Asian birds is explained by a single climate variable: sea surface temperature in the eastern equatorial Pacific in the previous fall. It is the rise in sea surface temperature in this region, more than 8000 km from Attu, that characterizes the onset of an El Niño episode. Examining those years for which there was a strong ENSO signal in the fall, it is found that the following May is characterized by anomalously strong westerly winds in the northwest Pacific, conditions that are appropriate for large Asian bird fallouts at Attu. Because of the time lag between the fall sea surface temperatures in the El Niño region and the spring Asian bird count at Attu, and the strong correlation between these two quantities, the number of Asian birds arriving at Attu in spring is predictable in the previous autumn. Such predictions are presented for several years.