Impacts of El Niño‐Southern Oscillation Events on the Distribution of Wintering Raptors

Abstract

Abstract We report the effects of El Niño‐Southern Oscillation (ENSO) events on the distribution and abundance of 3 raptor species at continental, regional, and landscape scales. We correlated values from the southern oscillation index (SOI), an index of ENSO phase and strength, with Christmas Bird Count data over a 30‐year period. We investigated the relationship between the SOI and winter raptor distributions at 3 spatial scales: continental (central United States), regional (TX, USA), and landscape (3 roadside transects within TX). At the continental scale, ENSO events resulted in regional shifts for American kestrel (Falco sparverius), northern harrier (Circus cyaneus), and red‐tailed hawk (Buteo jamaicensis) winter abundances. As expected, these shifts were northward during El Niño (warm) winters, and southward for red‐tailed hawks and northern harriers during La Niña (cold) winters. Within Texas, northern harrier distributions shifted towards arid west Texas during wet El Niño winters but were restricted to mesic coastal Texas during dry La Niña winters. Red‐tailed hawk abundance increased in eastern Texas during La Niña winters responding to cooler than normal temperatures throughout the northern Midwest. Data from local roadside transects over a 3‐year period encompassing 2 El Niño winters and one La Niña winter supported the abundance patterns revealed by continental and regional data, and added evidence that fluctuations in winter abundances result from demographic pulses as well as spatial shifts for wintering populations. This study underscores the need for long‐term monitoring at both local and regional spatial scales in order to detect changes in continental populations. Short‐term or local studies would have erroneously assumed local population declines or increases associated with ENSO events, rather than facultative movements or demographic pulses supported by this study.