In late April 2014, a massive storm swept through the southern United States, spawning 84 tornadoes, killing 35 people, and destroying more than $1 billion in property. But a batch of migratory birds in eastern Tennessee sensed the oncoming melee and left long before the first clouds arrived, researchers report online today in Current Biology. A year earlier, the team had tagged golden-winged warblers (Vermivora chrysoptera, pictured) with leg trackers in order to study their annual migrations to South America. Two days before the storms struck, five golden-winged warblers skipped town, traveling nearly 700 kilometers south to the Florida coast. (One bird flew as far as Cuba.) The birds had just finished a 5000-kilometer flight from Colombia 2 weeks earlier, suggesting that fatigue didn’t keep them from an impromptu redeparture. It’s the first time birds that normally migrate with the seasons have been shown to skip town when a big storm hits. The researchers suspect this behavior occurs only when the threat of injury outweighs the energetic costs of a long trip. But how did the birds guess the storm’s severity and leave so soon? The scientists initially reasoned that subtle changes in weather—atmospheric pressure, temperature, wind speed, cloud cover, or rainfall—signaled the pending disaster. Yet when they checked weather records, none of these factors fluctuated significantly prior to the storm. Instead, the team posits that the approaching storms created a tumult of infrasounds—low-frequency sound waves that birds hear, but humans can’t. Tornadoes produce infrasounds that can travel thousands of kilometers, so the birds may have heard the storm as it crossed a neighboring state, and then took off.
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