North American Birds

VOLUME 70 NO2 2018

A Quarterly Journal of Ornithological Record Published by the American Birding Association. The mission of the journal is to provide a complete overview of the changing panorama of our continent’s birdlife.

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140 N O R T H A M E R I C A N B I R D S The Changing Seasons: Winter of Insectivores The Changing Seasons: Winter of Insectivores MIKE HUDSON • 928 S CONKLING STREET • BALTIMORE, MARYLAND 21224 • (MHUDSON@ABA.ORG) W hen I sat down to begin work on this issue of North American Birds, I found myself thinking back to the winter of 2015–2016. I spent much of January of this year in Nicaragua for a tropical ecology course. We had been having an exceptionally mild winter. When I departed from Baltimore, Maryland, it was scarcely cold enough for a jacket; I recall that I left the house in a t-shirt and a light fleece. When I returned, it was several degrees be- low zero, there was snow on the ground, and a blizzard was expected to drop nearly a foot of snow on central Maryland over the next day-and-a-half. Talk about a welcome home! It is difficult and inaccurate to point to black-and-white divisions when looking at changes in climactic conditions or even general weather patterns. NAB readers are not likely to need a lesson in the many com- plicated and contradictory forces control- ling our climate. However, when I sat and thought about it more, I de- cided that what stuck out to me the most about the winter of 2015–2016 was how, more than other winters I can eas- ily recall, it was a marked by a pattern of long-staying mild weather punctuated by bitter, later-season winter storms. Perhaps part of this is a result of my young age; no doubt many of you with longer memories may not be struck as much by this was I was. Still, as you read on, you will see that my impression is not an isolated one. Many regional reports make note of the record-breaking warmth their regions experienced. This unusually mild weath- er brought with it a spate of somewhat predictable late- season species, along with several more unexpected ones. Late warblers were widespread: a Pine Warbler in Saskatchewan and Yellow-throated War- bler in Utah were notable finds among that family. Other birds that are somewhat antici- pated when the weather remains mild, in- cluding House Wrens and orioles, were also widely reported. Totally unanticipated, how- ever, was the appearance of multiple White- throated Swifts in Colorado. While many of the unusual overwintering migratory land- birds show some plasticity in their diet when pressed—I think I've now seen photos of nearly a dozen species of wood-warblers on suet feeders in winter—swifts are a notable exception in that department. One benefit to writing this two years af- ter the fact is that I can look back and say that this did not appear to be a one-off event. Both years since, I read many reports of overwinter and late-departing warblers, vir- eos, and other passerines. And the following year, not only were White-throated Swifts still in Colorado through the winter, but an- other unexpected species joined them—Say's Phoebe's were widely reported throughout Colorado, starting with onset of the Christ- mas Bird Count season. CBC circles that had previously been lucky to average one every few years were suddenly putting up multiple individuals. The evident increase in wintering popu- lations of obligately insectivorous bird spe- cies should be of great interest to the student of avian S&D—especially inasmuch as this event from the winter of 2015–2016 seems to part of a longer-term phenomenon. There is still a great deal that can be learned about what exactly is going on. Field ornitholo- gists are in the stage of learning what species are affected and exactly why. Are there spe- cific insects species fueling these changes? What happens when individuals get caught in bitter, late-season storms? Anecdotally we know some stick it out, but how many? What about the ones that don't?—Do they die, or do they engage in facultative mid-winter move- ments to warmer or otherwise more favorable climes? My impression is that the con - ventional wisdom favors the former hypothesis; but I am also intrigued by recently ac- cumulated for more extensive winter dispersal than we had previously appreciated. As long as these questions are in need of answering, there will be a need for an outlet such as this one to air those discussions. I firmly be- lieve that readers of NAB are well-equipped to begin inves- tigating not only this particu- lar topic, but so many others pertaining to avian S&D. My fervent hope is that, at some point in the not-too-distant- future, we may have the op- portunity to include notes and articles on some of these very topics. n Utah's sixth Yellow-throated Warbler and first since 2004 was discovered 25 December (here) at Logan, Cache County. This species is among the insectivorous species which exhibit some plasticity in diet when overwintering well outside their normal ranges. This cooperative individual continued visiting feeders and delighting birders through 5 January. Photo by © Dale Ashcroft.

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