How Birds Survive Winter: Torpor, Hibernation, and Migration

As winter storms still rage across parts of the United States, birds are preparing to move north for spring migration. As temperatures finally warm up, we wonder about how our feathered friends spent the cold, harsh winter months.  When temperatures drop and food becomes scarce, animals have different ways to survive. Some go into hibernation, while others enter a state called torpor. Though they might seem similar, there are key differences between the two. Birds don’t hibernate in the way that mammals like bears do, but many species experience torpor to get through cold nights or periods of food scarcity.

Torpor is a short-term, reversible state of reduced metabolic activity. During torpor, a bird’s body temperature, heart rate, and breathing rate drop significantly, allowing it to conserve energy. This state usually lasts for a few hours, or overnight, helping birds survive cold nights or food shortages. Because torpor is temporary, birds can wake up and become active relatively quickly when needed.

Hibernation, on the other hand, is a prolonged state of dormancy lasting for an extended period, often weeks or months. In hibernation, metabolism slows even more drastically than in torpor, and the animal remains in a low-energy state for an extended time. Many mammals, such as bears, bats, and ground squirrels, hibernate to survive winter conditions. 

Many species rely on torpor to get through difficult conditions. Hummingbirds are among the best-known examples. These tiny birds have incredibly high metabolisms and need to consume a lot of food to maintain their energy levels. When temperatures drop at night, and food is unavailable, hummingbirds may enter torpor to conserve energy. Their body temperature can drop dramatically, sometimes by nearly 50 degrees Fahrenheit, allowing them to survive until morning when they can start feeding again. Ruby-throated Hummingbirds are also thought to use torpor as a strategy to conserve energy for future migration, as a way to save up fat stores for the long journey ahead.

Other birds that use torpor include nightjars, swifts, and chickadees. Black-capped Chickadees, for example, enter torpor on especially cold nights, lowering their body temperature by about 20 degrees to reduce energy loss. This ability helps them survive harsh winter conditions in northern climates. 

One of the most persistent bird myths involves swallows and hibernation. For centuries, people believed that swallows spent the winter hibernating underwater! This idea dates back to ancient times when early observers noticed that swallows disappeared during the colder months and assumed they buried themselves in mud beneath lakes and rivers until spring. Some even claimed to have seen swallows emerging from the water when the weather warmed.

In reality, like many species, swallows migrate to warmer climates instead of hibernating. Barn Swallows, for example, travel thousands of miles from North America to Central and South America (or from Europe to Africa -- these birds are fairly cosmopolitan). The myth of swallows hibernating underwater has long been debunked, but it remains a fascinating example of how people once tried to explain mysterious bird behavior before the science of migration was well understood.

Torpor is a crucial survival strategy for many birds, allowing them to conserve energy during cold nights or periods of food scarcity. It may also be used to save up energy for future use, even when weather conditions are mild.  Torpor provides a temporary solution that helps species like hummingbirds, chickadees, and nightjars endure harsh conditions.  Whether by entering torpor or flying thousands of miles to warmer climates, birds have developed remarkable adaptations to make it through challenging seasons.