Yesterday was one of those unseasonably warm February days so, with coffee cups in hand, we settled in to watch the birds at the feeder. A week earlier, when the temperatures were in the mid-20s, the birds were pushing and shoving, trying to get a place to perch on the feeder. Today, though, there was almost no activity. Why, I wondered, did the birds choose to skip a meal just because it was warm outside?
It turns out the answer is a bit complicated. Birds – especially the small songbirds attracted to our feeders – burn a lot of energy. Scientists have calculated how much energy they need and call it the basal metabolic rate (BMR). It turns out that the smaller the bird, the more energy they need. Hummingbirds have a BMR of 1600 kilocalories per kilogram of body weight per day. House wrens, another very active little bird, have a BMR of 580 while white-crowned sparrows come in at 324. Big birds such as trumpeter swans come in at 47 while brown pelicans have a BMR of 75. Chickens – at least Rhode Island Reds – come in at around 56 BMR.
Birds have a longer evolutionary history than mammals, originating as an offshoot of dinosaurs more than 150 million years ago. With this long evolutionary history — and early start at living on our planet — they have developed several unique adaptive mechanisms for dealing with an array of environmental conditions. Because they don’t sweat, temperature regulation is different in birds.
First off, they are very efficient at converting food into energy. The transformation is fairly quick, so on cold days these little, highly active songbirds with high basal metabolic rates must eat a lot to keep their body temperatures up. And their body temperatures are higher than most other animals, averaging around 104 degrees F. Going into a long night of roosting in cold weather, black-capped chickadees can accumulate as much as 10 percent of their body weight as fat to keep their high metabolic rate going through the night.
We’ve all watched the bullies of the bird feeder chase off their own kind and sometimes, the smaller birds too. This pecking order – at least within a species – has some unusual effects. It turns out that the dominant bird weighs less than the others further down in the pecking order. These dominant individuals eat whenever they want, but they usually eat less during the day because they are watching out for predators. The less-dominant individuals are actually fatter (and slower if a predator arrives), but the dominant individuals arrive late in the day and fill up for the night so they will have more energy reserves and be more alert through the nighttime.
On our warm February day, the birds did not need to feed to keep their body temperatures up to battle cold, so they skipped breakfast. Had they eaten their fill, they would have had to resort to cooling techniques such as panting to disperse the extra heat burden they carried. With birds, it seems they eat what they need in relation to the environmental conditions they are dealing with. They have perfected some interesting techniques for dealing with cold.
Watching the sandhill cranes standing one-legged in the cold water of the North Platt River a couple of years ago, I marveled at their toughness. It turns out bird legs have a circulatory system designed to transfer heat from the warm incoming blood in the arteries to the cold blood returning from the feet in the veins, thus maintaining the core body temperature of the bird. And by standing on only one leg, the bird reduces by 50 percent the heat loss that does occur.
Another adaptational feature that occurs is called avian hypothermia or torpor. While not found in all birds, many small songbirds can drop their body temperature as much as 18 degrees F at night, thus reducing the need for as much energy during the coldest period. The dominant birds that eat more — and eat later in the day — don’t slip as deeply into this state of torpor, so are more alert and less likely to be eaten by the family cat. This adaptation may be an echo from the evolutionary past when birds branched off from dinosaurs, which many think were cold-blooded.
Fluffing feathers, huddling, and migration are all ways birds have adapted to survive cold conditions. While I missed watching the bird jostle for position on the feeder, I now have a better feel for why feeding time can be such a hit-or-miss proposition.