Deep time and the birds of our northwoods

The northwoods are home to birds that live here the entire year and those that are sensible enough to migrate south to warmer, if not tropical, climates. The evolutionary driver is food. The woodpeckers that are year-around residents of the northwoods can find food even on those short, brutally cold winter days. Black-capped chickadees store (cache) food and retrieve it during winter, roost in cavities or out of the wind, and lower their body temperatures to survive long winter nights. While our woodpeckers and chickadees are riding out the winter with its challenges, the blackburnian warblers that summer in the Northwoods are in South American mountain forests from Venezuela to Peru. In these southern reaches, blackburnian warblers can find food they couldn’t otherwise find in the northwoods winter. Of course, this isn’t simple, as there are new insects to eat, different plants, new competitors, and a multitude of predators (think snakes). Nonetheless the warbler is migratory whereas the chickadee is sedentary. Both strategies are adaptations to finding food, each with costs and benefits.
Speaking of winter, how did any birds survive 21,000 years ago, when at the Last Glacial Maximum, Minnesota was covered (mostly) by a mile-thick glacier? We might think winter can be brutal today, but nothing compares to that time. Obviously, all birds lived south of the glacier, but how far south? Reconstructions of vegetation at that time suggest that just south of the glacier there was “tundra-steppe,” which looks nothing like the northwoods today. Habitat similar to that in which some northwoods birds live today was compressed along the southern coast of the U.S. With so much water tied up in glaciers, sea level was about 400 feet lower than today, so for example, Florida was about twice as wide as it is now. However, the available habitat was a small fraction of what is available today.
My colleague Aubrey Gardner and I proposed that at the height of the last glaciation many migrant species, such as the blackburnian warbler, lived year-around in the tropics, where they now only winter. We titled our paper “Glaciation as a migratory switch” because we considered the beginning of a glacial advance as being a switch to a tropical sedentary lifestyle, and the retreat of glaciers as a switch back to a migratory lifestyle. One might wonder why these warblers didn’t remain tropical. It’s about food and raising young. Ornithologists think many of our species were originally tropical residents, but with the summertime establishment of the northwoods environment full of bugs, birds could raise more young by coming north to breed than they could by staying in the tropics, even given the considerable costs of migration. The two strategies take advantage of whatever was the most favorable at the time for raising the most young.
Is it a big deal for northwoods species to switch from sedentary to migratory? It might have been at first. Because there have been consistent cycles of glaciation in North America, migration and tropical sedentary “programs” probably have become engrained in their genomes. As a result, birds are adapted not to just seasonal winter, but glacial “winters” lasting 100,000 years. They just need the right switch, glaciation in our opinion.
This winter when you gaze outside at the relatively few birds at your feeder, don’t fret. The summer residents will return in spring, once the bugs are back, for the foreseeable future. But it’s also the case that many of our migrant birds will likely someday become tropical residents once again. Where we’ll be living is a good question.