On a trip to Washington this past February, it seemed strange to see (Anna’s) hummingbirds coming to the feeders. Here in Colorado, we aren’t so lucky. The species we enjoy here depart in the fall and don’t return until the end of April—or even later. Still, I’ll be brewing up some sugar water soon. I typically hang my feeders around April 25, just in case some early arrivals show up in the backyard. (When temperatures dip below freezing, I take the feeders in for the night, then warm them a bit for the birds’ breakfast.)
Over the years (at our previous house), I learned to expect our first Broad-tail on April 30—no matter what the weather. It could be warm and sunny, or we could be having a blizzard, and the bird (I’m sure it was the same one) would appear like clockwork.
Even if it isn’t snowing, nights in late April and early (or mid-!) May can be cold. It amazes me how something as tiny as a hummingbird can withstand such low temperatures. They just don’t have that much body mass, especially compared to their surface area. Moreover, hummingbirds lack the down that keeps many birds warm. Their sleek feathers alone can’t provide much insulation. They aren’t chowing down on high-energy food, either. There aren’t a lot of high-fat bugs to eat at this time of year. So how do these tiny birds survive when I’m huddling inside next to the fireplace?
It turns out that hummingbirds have a superpower. They can simply enter a form of suspended animation until temperatures rise again. This ability is called torpor.
Wikipedia describes torpor as “a state of decreased physiological activity in an animal, usually by a reduced body temperature and metabolic rate.” The idea is to save energy in order to survive periods of cold or insufficient food.
It turns out that hummingbirds aren’t the only animals that have this ability. Some other birds, such as swallows, swifts, and poorwills, can also lower their metabolism. So can bats, mice and other rodents, and marsupials. “Daily torpor” (also called “noctivation”) might last just for the night. When torpor lasts for months, we call it hibernation. Hummingbirds can enter torpor at any time of the year, whenever temperatures warrant.
During torpor, the birds lower their temperature until it barely sustains life, far below their normal body temperature of 104°F. This lowered set point is carefully monitored by the bird’s internal thermostat.
To the outside observer, they seem dead. Their eyes are shut, they don’t appear to breath, and poking them doesn’t wake them. They may even hang upside down on their perch. Clearly, they’re at a major disadvantage should a predator notice them.
An April 9, 2006 post by GrrlScientist explains how the birds wake up:
Awakening from torpor takes a hummingbird approximately 20 minutes. During arousal, heart and breathing rates increase and hummingbirds vibrate their wing muscles. Heat generated by vibrating muscles, or shivering, warms the blood supply. Shivering is sufficient to warm the hummingbird’s body by several degrees each minute and the bird awakens with enough energy reserves to see him through to his first feeding bouts of the morning. Interestingly, hummingbirds reliably awaken from torpor one or two hours before dawn without any discernible cues from the environment. Thus, it appears that the bird’s internal circadian clock triggers arousal.
Hummingbirds already impress us with their amazing abilities— migrating long distances (some Ruby-throated Hummingbirds manage to cross the Gulf of Mexico twice a year!), hovering in mid-air, flying backwards. The smallest bird in the world is a hummer—Cuba’s Bee Hummingbird is a mere 2.25 inches long and weighs less than a dime! Add in the ability to survive a blizzard, and they certainly make my list of most incredible animals ever!
The answer to last week’s bird quiz is American Avocet.