Shortly before midnight on the New Year’s Eve of 2010, the small town of Beebe, Arkansas watched as thousands of red-winged blackbirds fell from the sky. According to observers, dead blackbirds rained onto rooftops, sidewalks, and into fields. Wildlife mortality experts later determined that the event, dubbed the “aflockalypse,” was the result of an untimely fireworks display that lead to the loss of nearly 5,000 individuals as the startled birds collided with buildings and other obstacles. The catastrophic loss is an example of what scientists refer to as a “mass mortality event,” which can affect thousands, and even millions, of animals. Disease outbreaks, severe storms, droughts, human activities, or a combination of these factors can trigger an incident. While disturbing, they’re not always cause for concern—all major animal groups on all continents have experienced die-offs. But they may be occurring more frequently and affecting more individuals than in the...