Butterflies In December

by Martin Bialecki

s the temperature nudged the 70-degree mark, setting a new record for Dec. 5th, I was driving along the River Raisin’s back roads, noticing small yellowish butterflies commonly called sulfurs. On the radio the meteorologist announced that November was the warmest on record. In my consternation I yearned to find elucidation. The oft-cited law of probabilities, which states that the longer data is compiled the less likely it is for new records to be set, seemed as frail as those little yellow wings.

I wonder: is it my acculturated appetite for sensational extremes that heightens my awareness or is it my guilt-ridden greenness? However, temperature data demonstrates that the globe is in fact warming.

Butterfly records are not nearly as deep or common as temperature records. Yet a look at the relatively shallow records of “first and last dates” clearly indicates that records are being broken almost annually. One of the reasons for this is the increasing number of observers and their inclination to record extreme dates. But before we jump to any conclusions based on these data, we must look deeper.

Accounts of certain species stirring during the dormant season are well established in folklore. The occasional Mourning Cloak rousted briefly by a mid-winter thaw is one example. These are adults that were dormant and, barring mishap while sleepwalking, will likely return to dormancy. December sulfurs on the other hand are adults newly emerged from a chrysalis stage that normally stays dormant until spring. These mistaken emergers will perish as soon as temperatures dip to seasonal levels.

To understand the effect these weather changes have on different butterfly species we need to consider the differences between the two basic butterfly groups: residents and migrants. Residents are divided into two subgroups: the hardy ones that easily survive winter as adults (like the Mourning Cloak which can also migrate); and the less hardy ones that survive only in chrysalis (such as sulfurs), egg or larval forms. Then there are the Migrants, also divided into two subgroups: those that travel considerable distance and expend enormous energy breeding a new generation (most notably the Monarch); and those that visit but do not breed here (such as the Variegated Fritillary).

Confusing these distinctions are the “50/50” bugs such as the Red Admiral. The individuals observed earliest in the spring have arrived as migrants from the “near south” where they have just emerged from chrysalis stage or over-wintered as adults. There is increasing evidence that a limited number of both life stages are able to over-winter here. The earliest observations of Admirals point to the likely expansion of their “near south” range. As records for highest average temperature are broken this is a logical expectation. The “near south” is gradually spreading to the River Raisin basin.

With the continued rise in global temperatures it is probable that more species will shift into better-suited ranges. This is a double-edged sword. Not only are certain southern butterflies, previously considered rare visitors, going to become more common here, but others we have found here at the southern limit of their range are likely to shift northward and will be observed here only rarely. Much of this argument is conjecture, but it is grounded in the idea that trends are predictable. As we become more engaged in understanding our environment, we must also appreciate the value of good record keeping (even those seemingly minor nuances) because it is from these observations that the bigger picture begins to emerge. Roger Kuhlman is the foremost authority on local butterflies and maintains a comprehensive website of butterfly records at by clicking here.

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