As far back as the early 1900s, moose biologists had noted that late-coming warm winters, such as that of 2002, sometimes led to tick irruptions that killed young moose. But such winters were rare. Like droughts, then, these weather-related tick irruptions fit into a pattern of rare stressors that had little long-term impact.
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To the dismay of [researchers], such winters became far more common just as moose were growing denser than ever through much of their upper New England range. In the mild, late-coming winters of 2008 and 2011, moose biologists in all three northern New England states, now on the alert, saw signs of more ticks and higher calf mortality. By 2013, New Hampshire and Maine had started an updated, five-year version of the earlier winter moose-collaring study, which Vermont joined in 2015. It is for this study that [scientists] are doing field autopsies on dead, collared moose calves.
Before every one of the study’s first three winters (that is, the winters that carried into 2014, 2015, and 2016), the autumn was warm and short on snow; and at the end of every one of those winters, mainly in April, ticks sucked the life out of more than half of all collared calves in all three states. In 2016, when much of the study area had received very little snow before January, 80 percent of the collared calves succumbed.
