They should all be dead. The hummingbirds.
Two species of hummingbirds are common in the Rogue Valley, Anna's and rufous. The rufous hummingbird takes off for Mexico in winter, leaving by early October and returning in March. If you survive on flower nectar, heading to Mexico makes sense. Just try to gather a bouquet here in the Rogue Valley in December. Yet, the second hummingbird, the Anna's hummingbird, remains year round. Foolhardy, not to mention potentially fatal.
Anna's hummingbirds didn't always live here. They expanded their range into Oregon in the 1940s due to the proliferation of hummingbird feeders and homeowners cultivating plants with a longer flowering season. Still, there are great risks heading north for one so small. Not only is there a shortage of flowers in winter, but it often gets cold — really cold. And sometimes the cold lasts a long time.
The past couple of weeks brought us arctic weather and a fair amount of snow, and it lingered for nearly two weeks. During that time I found any number of reasons to stay inside. To venture out was to experience chilled cheeks, nose, toes and fingers even when bundled in winter clothing.
Still, humans are much better equipped to deal with the cold than hummingbirds. We are large. And that means we have a favorable surface-to-volume ratio. This ratio is the relationship between surface area of the body and body volume. The larger the body, the smaller the ratio, and the easier it is to resist the cold.
Now, if humans were uncomfortably cold in the recent weather, what does it mean for hummingbirds? Being tiny, they have a most unfavorable surface-to-volume ratio. It is almost impossible for them to burn energy fast enough to stay warm enough to survive. They must tank up on sugar water or nectar all day every day to lay down a thin layer of fat that will see them through the next night without starving. Still, by itself this is not enough. They also go into torpor, a sort of short-term hibernation in which they let their body temperature drop to between 47 and 68 degrees, well below their normal temperature of 104. This allows them to conserve enough energy to make it through the night. They do this even when it's relatively warm out.
What about several bitterly cold days and nights in succession? It takes huge amounts of energy to survive. It's worse when you consider that they wake up to find all of the hummingbird feeders a block of ice. Cold and no food. This sounds like a death sentence. OK, when a hummingbird feeder freezes, the sugar is the last to freeze. Usually there is a syrupy film remaining on the ice. Then there are sap wells left by sapsuckers and maybe fruit mummies remaining on some trees.
This all helps. But given the extended cold spell, they should all be dead. There is no way they could find enough food or conserve enough energy to survive. Yet, now that the deep freeze has departed, my hummingbird feeder has just as many clients as always. There are still plenty of mysteries remaining for scientists to solve, and this is one.
My hat's off to the tough little hummingbirds.
Stewart Janes is a biology professor at Southern Oregon University. He can be reached at firstname.lastname@example.org.