Bats: Do They Hibernate or Migrate in the Winter?


Is the weather getting chillier, and you are wondering whether you should start worrying about bats seeking shelter in your new attic? Have you noticed a few bats looking hungry flying around your porch? Does the end of summer mean that you will soon have to deal with a colony of sleepy mammals?

Depending on the species of bat in question, they either hibernate for up to six months or migrate to warm areas that offer higher availability of food. Most of the 47 species of bats that populate the U.S. can modify their migration strategy. 

While you might find bats’ winter tendencies fascinating and peculiar, it is essential to understand their migratory or hibernation patterns. This knowledge might help you prevent a colony from getting too comfortable in your living quarters! Check out the guidelines below to learn how to “bat-proof” your house!

Hibernation vs. Migration

During the hunting season, bats consume a significant amount of energy, flying around and trying to catch insects such as moths, beetles, or spiders. At the end of summer, the amount of edible insects lowers. 

As winter arrives, most mammals in the wild need to take precautions to survive the harsh months of scarcity in the wild. The lack of food, freezing temperatures, and low protection against the elements can represent a considerable threat. 

Bats tend to have two different solutions to this issue: hibernation or migration.

  1. Hibernation in bats is referred to as “torpor,” a sleep-like period in which moments of arousal are common. For their hibernation period, bats pick a hibernaculum, literally a “tent for winter” 1. Little Brown Bats are some of the most common species that you might find flying around your property. They can slow their metabolism down and hibernate for up to 6 months at a time. Such bats might pick your attic or barn as their hibernacula because of the high accessibility, protection, and warmth.
  • The majority of species of bats that tend to roost in trees prefer to migrate. Since they might struggle to find a shelter shielded from the elements in the outdoor environment, they simply decide to move to warmer areas. Among those species, you might find the less-common hoary bats. 

In some cases, they might pick a combination of the two solutions, flying to slightly warmer areas and still hibernating during the harshest, but shorter, periods. 

The choice entirely depends on the species of bat in question as well as the roosting space. Bats typically don’t migrate as far as certain species of birds do. Therefore, if their starting point is located too far from warmer climates, they might choose to hibernate.

Bat Hibernation

Similar to other mammals and most species of birds, bats are endothermic 2. This term refers to their ability to regulate their body temperature and retain heat in cold climates.

However, due to the lack of insects to feed on, most bats prefer to hibernate. This strategy allows them to lower their internal body temperature to match the outside climate. 

At the same time, mammals tend to burn a significant amount of energy just by breathing, pumping blood through the body, flying, digesting, and hunting. Due to the lack of food that could provide enough energy, some types of bats decide to hibernate for up to 6 months at a time. 

During the torpor months, their metabolism slows down 3. Alongside a low heart rate of around ten beats per minute, in this way, bats can control the consumption of stored fat.

Indeed, they can survive winters with just a few grams of it. It is not uncommon for hibernating mammals to lose up to half of their body weight during the dormant months.

Hibernating is essential for bats that find themself too far from any warmer locations.

Developing factors such as climate change and white-nose syndrome can interrupt the hibernating patterns and eventually be extremely detrimental to their health 4.

When do bats hibernate?

Bats’ hibernation period starts in late autumn, around the end of October or the start of November. The dormancy can last up to six months, until the arrival of springs, in March or April. 

While this is a general timeframe, torpor patterns in bats vary significantly across species and, sometimes, across individuals. While hibernation is commonly known to be a 6-months long sleep, moments of arousal are frequent and can last up to 24 hours 5.

During hibernation, bats decrease their body temperature and, since the immune system is particularly energy-consuming, it is one of the first to lack resources. To preserve their wellbeing through 6 months of inhibited immune systems, bats need to wake and restore their higher body temperatures at regular intervals.

Where do bats stay and hibernate?

The six months of hibernation are among the most threatening periods of their lives. They are often fatigued, low in energy, food-deprived, and with a weak immune system. 

Therefore, before falling into their torpor status, they tend to pick the ideal spot to spend the following six months. These places, or hibernacula, can be different and vary from one year to the other. 

However, they must all provide essential features:

  • Ideal temperatures – Even though bats can control their body temperature and avoid freezing, they require specific temperatures for hibernating safely. The ideal estimation is between 1C and 5C (35F to 40F). Since bats tend to keep their internal temperature only 1 to 2 degrees Celsius above the external climate, any temperature below the ones mentioned can cause them to freeze. At the same time, hotter external conditions can require them to waste too much energy.
  • Protection from elements and predators – As we have seen, bats are at their most vulnerable during the hibernation period 6. Conditions such as White-nose Syndrome and predators can lower their survival chances 7. Therefore, a bat will always look for an enclosed, protected, and hard-to-find hibernacula.
  • Direct access to outdoors – Have you ever wondered how bats know when it is time to wake up? Their hibernacula’s entrance draws in the airflow from outside, so bats can gauge the temperature difference and leave their shelter in the spring.

Don’t forget that bats acclimatize easily to the urban environment. Therefore, they are extremely likely to pick as their hibernacula a chimney, vent, or siding rather than a cave. The least crowded or busy areas of our homes feature all these characteristics while being close to their natural habitat.

What precautions should you take?

As winter approaches, bats will be on the lookout for a crack or gap to enter a warm and protected hibernaculum. 

At the end of summer, you should ensure that no bats have direct access to your living quarters. To do so, inspect your house’s roofing, walls, ceiling, sidings, fascia board. Any whole that is larger than ⅜ of one inch will allow for bat entry. 

Once the bats have colonized your house, it can be challenging to eradicate the issue. Bats won’t fly all out at the same time, so you might find it impossible to seal the entrances later on. Prevention, in this case, is critical.

What species of bats hibernate?

As we have seen, bats hibernation patterns vary depending on the species, colony, and even individual preferences. 

However, the most likely type of bat that can end up populating your attic is Small Brown Bats. These are extremely common in many households and tend to hibernate in colonies alongside other species such as Soprano Pipistrelle bats.

Bat Migration

Migration is another adaptation strategy that certain types of bats might opt for to survive the winter. This term refers to the ability of mammals to move between a winter and a summer habitat. 

This strategy guarantees them to have the availability of food they need all year round. Other reasons, such as finding a place to hibernate and then raise offspring during the warmest months, can also drive migration.

However, migratory behaviors come with crucial downsides that bats have to consider before flying off. Firstly, flying to warmer climatic zones is only viable if these are within the bat’s flying radius. Even in this case, bats might need to fly for hundreds of miles before reaching a viable spot.

Secondly, bats, similarly to other birds and mammals, have to prepare for migration through several physical adaptations such as:

  • Fat increase and deposition
  • Improve muscle capacity
  • Essential changes in the digestive system 8

Studies have also proved that Big Brown Bats use the magnetic field of the Earth as a reference point for orientation, and they might adjust their trajectory based on the sun. Lastly, even though they are nocturnal animals, bats can fly in a group during the daytime as well.

Bats migration patterns

Bats can fly at a speed as high as other birds, yet they won’t cover extended distances. Based on other studies conducted on the migratory patterns of birds, researchers have found that bats follow the same natural cues to orientate themselves, prepare in a similar way for the flight, and use airflow to cover more considerable distances 9.

Some species of bats can even fly distances of up to 200 miles 10.

The reason why bats take much longer than other birds to cover longer distances resides in the fact that bats are nocturnal animals. A bird will fly during the day and forage food at nighttime or vice versa. However, a bat completes all its tasks overnight. Therefore, it will need to split those hours between flying and hunting for food.

Due to the limited timeframe and short distance that they can cover in a day, bats tend to prefer to migrate over hibernation only if they roost near warmer climate zones. 

What species of bats migrate?

Generally, the species of bats that prefer to migrate instead of hibernating are the ones that roost on trees or outdoors. Bats that live in urban environments such as cities tend to roost in warm chimney and vents. When winter comes, they won’t find it hard to settle in an enclosed, perfectly-conditioned hibernaculum.

Instead, in natural environments, finding an adequate hibernaculum that is both warm and provides protection can be tricky. While trees and caves can offer little protection from the elements, they might not reach the minimum temperatures bats need to hibernate safely. In this case, bats are incredibly likely to move towards warmer areas where they can settle outdoors without feeling threatened.

Other adaptation strategies

Some bats opt for a combination between hibernation and migration. This choice depends on their starting point and roosting habitat. It mainly happens if bats find themselves in the impossibility of finding adequate hibernacula while far away from any viable hotter climate zone.

In this situation, they will fly toward a warmer area and then stop to hibernate. Indeed, the long migratory flight can fatigue bats, deprive them of essential food intake, and reduce their energy levels. As they find a warm and protected location, these bats will enter a shorter hibernation period. At the beginning of Spring, the hibernation place will become a roosting habitat to mate and give birth to offspring.

In specific cases, the aim of the migration itself is to reach the hibernacula, which is static and in a warmer zone. While this is a just-as-viable migratory pattern, only bats that tend to avoid roosting in urban areas will follow this returning pattern. 

Ultimately, bats are always after the perfect living and roosting conditions. If they find them in the place where they usually dwell, they won’t need to find other adaptation solutions such as migrating or hibernating. For example, bats living in Florida won’t ever need to leave to survive!

When is your house more at risk?

Undoubtedly, the best way to deal with a bat problem in your house is to prevent their entrance altogether. 

While you might tolerate one or two bats living in an unused barn, a colony living in your storage place can represent a health and safety issue for you and your family. To avoid excessive frustrations, understanding bats’ migrating and hibernating patterns can offer you a clue on when you should be the most careful.

Fall

Fall is the most critical time of the year as, during these months, both migrating and hibernating bats are on the lookout for their next hibernaculum. Especially between October and November, wildlife-protection organizations and experts suggest “bat-proofing” both you and your house.

To do so, examine your house and identify all the entrances that could be an accessible way in for bats and other pests. Moreover, you can consider implementing chimney and vents covers and nets. These simple preventive measures will help you prevent a colony of bats from settling in your home.

Spring

The pupping season for bats ranges between June and August. Each female will give life to one pup on average per year. Once a maternal colony has given birth to offspring, the removal of both bats and their roosting place can be extremely challenging, as you will need to care for the safety of the newborns as well.

In Spring, migrating bats are on the lookout for a place to settle, mate, and give birth to their little ones. Preventing their entrance at this time of the year is essential. If you have put precautions in place during fall, ensure that they are still valid.

Sudden changes in temperature

Whether you might have inadvertently caused it or it derives from an unexpected turn of weather, any sudden peak or drop in temperatures can wake bats during their torpor period. 

Since the roosting habitat is extremely important for the survival and wellbeing of bats, they might wake up and search for a more comfortable place to live. It is not unlikely that this might be directly within your living quarters. 

If you find bats droppings by a barn or attic, you should get in touch with an organization that deals with wildlife protection. Handling bats yourself can be dangerous for both your and the safety of the bats. 

To wrap things up

Bats are often an annoyance for scared house owners. Indeed, they should never enter your living quarters to guarantee high levels of hygiene in your house. 

However, bats are on a constant quest to find the perfect roosting place. Bats that live in urban areas won’t be able to survive the winters if left outdoors, and they will, therefore, try to find the perfect hibernacula in your chimney or sidings.

Oppositely, tree-roosting bats tend to migrate to warmer areas to find appropriate habitats. Whether they decide that your attic is their preferred place to roost or they just fly into your living room by mistake, you should always treat bats with caution, humanity, and compassion. 

References

  1. Klüg-Baerwald, B. J., Lausen, C. L., Willis, C. K., & Brigham, R. M. (2017). Home is where you hang your bat: winter roost selection by prairie-living big brown bats. Journal of Mammalogy98(3), 752-760.
  2. Holyoak, G. W., & Stones, R. C. (1971). Temperature regulation of the little brown bat, Myotis lucifugus after acclimation at various ambient temperatures. Comparative Biochemistry and Physiology Part A: Physiology39(3), 413-420.
  3. Geiser, F. (2004). Metabolic rate and body temperature reduction during hibernation and daily torpor. Annu. Rev. Physiol.66, 239-274.
  4. Falvo, C. A., Koons, D. N., & Aubry, L. M. (2019). Seasonal climate effects on the survival of a hibernating mammal. Ecology and evolution9(7), 3756-3769.
  5. Luis, A. D., & Hudson, P. J. (2006). Hibernation patterns in mammals: a role for bacterial growth?. Functional Ecology, 471-477.
  6. Bernard, R. F., & McCracken, G. F. (2017). Winter behavior of bats and the progression of white‐nose syndrome in the southeastern United States. Ecology and evolution7(5), 1487-1496.
  7. https://www.usgs.gov/faqs/what-white-nose-syndrome
  8. Fleming, T. H., Eby, P., Kunz, T. H., & Fenton, M. B. (2003). Ecology of bat migration. Bat ecology156, 164-65.
  9. Hedenström, A. (2009). Optimal migration strategies in bats. Journal of Mammalogy90(6), 1298-1309.
  10. http://publichealth.lacounty.gov/vet/rabiesmanualpdfs/rabies_faq.pdf

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