For the Birds Radio Program: Redpoll
One of the tiniest birds to visit the north woods in winter can survive colder temperatures than any other songbird, at least based on laboratory tests. Decades ago, before such experiments became illegal, scientists found that Common Redpolls can survive in a chamber for three hours at -65 F, and that Hoary Redpolls can survive that long when the temperature is kept at the almost unearthly temperature of -88 F. Of course, those experiments were more cruel than the findings were interesting—nature is far more yielding than a laboratory chamber. Redpolls, chickadees, and other birds in Tower, Minnesota, survived the night of February 2, 1996, when the temperature dipped down to 60 below, but many of those birds spent the night inside cavities in thick trees, their own bodies and, in some cases, the warm bodies of their companions, raising the temperature to more survivable levels. Other birds buried themselves under snow or snuggled next to one another on sheltered branches. And first thing in the morning, their ability to fly here and there allowed their muscles to heat up their bodies more than mere shivering could—that isn’t an option in a laboratory chamber. I’m not sure that the stress of serving in an experiment to see how long it would take them to die didn’t hasten the process.
People hardly needed experiments to prove how hardy redpolls are. They nest to the farthest northern reaches of Alaska and in much of northern Canada, and many don’t retreat much further south in winter. They have several habits and physical adaptations that allow them to survive extreme cold. They spend a lot of time on the ground, where they can find low areas more sheltered from wind than in trees. They gravitate to a very high-energy diet of birch seeds, and their digestive efficiency is better than most birds. Their feathers are structured to provide superior insulation, and they add more feathers when they need them—their plumage weighs 31 percent more in November than July. In the far north where winter nights are far longer than days, redpolls can eat in lower light than most songbirds because they have relatively more rods in the retina. And at their final feeding as night takes over, they can literally eat more than their stomach can hold, storing seeds in diverticula, expandable little pouches in their esophagus. Throughout a frigid winter night they can be digesting food to stoke their metabolic fire even as they sleep. During bad weather, redpolls are exceptionally fast eaters— when they must venture out to get food, they swallow seeds whole, stuffing their diverticula and then retreating to shelter to regurgitate and husk them.
The cost of being able to survive such frigid temperatures is a low tolerance for heat. When the temperature goes up, redpolls become less active, eat less, require more water, and start plucking their feathers. The highest temperature they can endure is about 100 degrees.
In some winters, exceptionally high numbers of redpolls visit the upper Midwest; other winters they’re almost non-existent. When redpolls are about at all, they tend to be seen in fairly large flocks. In a poor year, a handful of people report large flocks, and most others see none. In an invasion year, a great many people see large flocks. During January this year, not that many people were reporting them. Ironically, as winter starts releasing its hold, we often start seeing more and more redpolls. Sure enough, reports of them from delighted birders started appearing regularly in February, and the numbers of reports have been increasing in March. These acrobatic little wonders will stick around for the next few weeks, heading north again in April. They and Pine Siskins are extremely vulnerable to salmonella. Warm temperatures, the moisture from melting snow, and decaying seeds beneath our feeders work together to produce very unhealthy conditions for them. Shoveling or raking yucky seeds from below our feeders and disposing of them in well-covered or screened compost bins is essential if we don’t want to compete with the scientists of earlier eras in squandering these valuable lives.