We all feel a bit of slowdown in winter, when we might prefer to bundle up in a cozy quilt and just snuggle in for a nap until the snow melts. Many animals have similar preferences and will bundle up in their own fur coats, snuggle in to a warm den, and hibernate until spring. But what about birds?
Hibernating Birds and Torpor
Birds do not generally hibernate as many mammals and reptiles will. They can enter a similar state called torpor, when they slow their body metabolism, lower their body temperature, and conserve energy until conditions improve. This low-energy state typically lasts just a few hours or overnight. Birds will burn less body fat and consume less oxygen. Their metabolism might slow by as much as 95 percent depending on the bird species and the conditions it is facing.
See also: How Do Birds Survive Winter?
Birds use torpor most often during winter nights, when extremely low temperatures and scarcer food make survival more challenging. Unlike hibernation, however, torpor is not a seasonal phenomenon. Birds might use it at any time of the year when conditions are harsh.
The Hibernating Whip-Poor-Will
There is only one bird in the world that has been documented as using a stronger type of hibernation, similar to a mammal’s seasonal hibernation. The common poorwill (Phalaenoptilus nuttallii) does truly hibernate, entering a deep torpor state that may last for several weeks or months. This bird’s hibernation is not consistent, however. If conditions are favorable, the common poorwill won’t hibernate at all, or its hibernation will be much shorter or less dramatic.
Which Birds Use Torpor?
Certain birds are more likely to frequently use torpor. Smaller birds that generate less internal body heat or store less body fat will use torpor more often. As will desert birds that may be subjected to dramatic temperature extremes between night and day. Birds that rely heavily on insects for food are also more likely to use torpor overnight, when insects have vanished and the birds cannot feed until warmer daytime temperatures stimulate their prey.
Bird species known to use torpor include:
In addition to these birds, small owls will also occasionally use torpor on cold nights. Many other animals can also enter this semi-hibernation state, including bats, mice, badgers, hedgehogs, and dwarf lemurs.
Pros and Cons of Torpor
Torpor can be both useful and dangerous for birds. While in a torpid state, birds are using less energy and thus conserving body fat and calories for when they wake up and need to forage, defend their nesting territory, or continue a migration journey. This condition is also useful when food is scarce, and birds can survive longer on less food if they use torpor. Many birds that use torpor also remain in the same range year-round. It allows them to maintain the same prime territory without the risks associated with migrating.
See also: How to Feed Birds During Winter
When a bird is torpid, however, its senses are less alert and its reaction times are much slower. This sluggish behavior can make the bird more vulnerable to predators, particularly at night. Another risk is that conditions will not improve soon enough for a bird to recover from torpor, leading to the bird’s death when even its slower metabolism isn’t enough to withstand extreme cold or lack of food.
Waking up from torpor is a slow process that might take a bird several minutes or as long as an hour or more. First, a torpid bird will often start shivering, which will slowly raise its body temperature and heighten its senses. The bird may soon start looking around, and could stretch or reshuffle its posture to encourage better blood flow as its body metabolism returns to an active state. As soon as it has woken up, the bird will need to find a rich source of food to replenish its energy before resuming its normal activities.
See also: Where Do Birds Sleep at Night?
Torpor is just one fascinating way birds keep warm and survive harsh conditions. By better understanding this behavioral adaptation, birders will not only appreciate just how resilient birds can be, but will be able to provide safe spaces for roosting birds and rich foods to help them use torpor most efficiently.