Henry M. Streby, Gunnar R. Kramer, Sean M. Peterson, Justin A. Lehman, David A. Buehler, David E. Andersen

Migration is a common behavior used by animals of many taxa to occupy different habitats during different periods [1]. Migrant birds are categorized as either facultative (i.e., those that are forced to migrate by some proximal cue, often weather) or obligate (i.e., those that migrate on a regular cycle) [2, 3]. During migration, obligate migrants can curtail or delay flights in response to inclement weather or until favorable winds prevail [4, 5], and they can temporarily reorient or reverse direction when ecological or meteorological obstacles are encountered [6]. However, it is not known whether obligate migrants undertake facultative migrations and make large-scale movements in response to proximal cues outside of their regular migration periods [3]. Here, we present the first documentation of obligate long-distance migrant birds undertaking a facultative migration, wherein breeding golden-winged warblers (Vermivora chrysoptera) carrying light-level geolocators [7, 8] performed a >1,500 km 5-day circumvention of a severe tornadic storm. The birds evacuated their breeding territories >24 hr before the arrival of the storm and atmospheric variation associated with it. The probable cue, radiating >1,000 km from tornadic storms [9-11], perceived by birds and influencing bird behavior and movements [12-14], is infrasound (i.e., sound below the range of human hearing). With the predicted increase in severity and frequency of similar storms as anthropogenic climate change progresses [15], understanding large-scale behavioral responses of animals to such events will be an important objective of future research.