WildWatch Weekly

Bats on the Move: Migration and Hibernation in Pennsylvania

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If you’ve peeked into the Schwarz barn this month, you may have noticed fewer furry faces hanging from the rafters. All summer long, the barn has been alive with the flutter of wings at dusk as our big brown bat colony took off to hunt insects over the fields. Now, as the days grow shorter and the nights turn cooler, those familiar shapes are disappearing, and it leaves us asking an important question: where did the bats go?

The answer is one of nature’s most fascinating seasonal stories. Pennsylvania’s bats take two very different approaches to surviving the long, cold winter. Some choose to stay close to home, finding a safe, sheltered spot to hibernate until spring returns. Others take to the skies and begin a migration journey, flying hundreds of miles to reach warmer climates where food is still available.

This split strategy, hibernation versus migration, ensures that bats as a group can survive Pennsylvania’s harsh winters. By slowing their metabolism or by seeking out new hunting grounds, they continue their quiet but crucial work of keeping insect populations in check. Understanding which species do which, and when, helps biologists track population health and offers all of us a deeper appreciation for these underappreciated nighttime neighbors.

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Our Schwarz Barn Colony

All summer long, the Schwarz barn has been home to one of our favorite local wildlife spectacles, a thriving colony of big brown bats. At the height of the season, dozens of bats can be seen tucked into the rafters during the day, waiting for dusk to bring their nightly insect feast. On warm July evenings, the air outside the barn can feel almost electric as the bats pour out in a steady stream, each one playing its role in keeping moths, beetles, and mosquitoes in check.

But as September rolls in, those numbers start to drop. When we peek into the barn now, we’re seeing fewer bats than just a few weeks ago. Some individuals have likely moved on to other winter roost sites, but many will stay relatively close. Big brown bats are one of the few species in Pennsylvania that overwinter here rather than migrating long distances. They choose protected spaces like barns, attics, hollow trees, or even abandoned buildings where they can enter torpor, dramatically slowing their heart rate and conserving energy until spring.

Keeping track of how many bats we see each year is more than just a curiosity, it’s an important piece of citizen science. Big brown bats have been more resilient to threats like white-nose syndrome than many cave-hibernating species, but even their populations can fluctuate with changing weather patterns, habitat loss, or pesticide use. By monitoring the Schwarz barn colony and sharing what we see, we can contribute data that helps researchers understand population health and long-term trends. It’s one small but meaningful way we can help protect these critical insect-eaters.

Watch the Schwarz Bat Colony LIVE Here: https://pixcams.com/wlt-schwartz-barn-bat-colony-cameras/​

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The Hibernate-and-Wait Strategy

For many of Pennsylvania’s bats, surviving the winter means finding a quiet, dark, and stable place to wait out the cold. Species like big brown bats, little brown bats, Northern long-eared bats, tri-colored bats, and the federally endangered Indiana bat all rely on hibernation as their primary survival strategy. Once temperatures drop and insect prey becomes scarce, these bats retreat to caves, abandoned mines, deep rock crevices, and even human structures that stay cool but above freezing.

Inside these hibernacula, bats enter a state called torpor, dramatically slowing their heart rate, breathing, and metabolism. A bat that might have a heart rate of over 200 beats per minute while active can drop to just a handful of beats per minute in hibernation. This deep rest allows them to stretch their fat reserves through months when food simply isn’t available. They will only wake occasionally, using precious energy to shift position or drink water before settling back into their winter slumber.

Unfortunately, this hibernation strategy has made many of these species vulnerable to white-nose syndrome (WNS), a fungal disease that thrives in the same cool, damp cave environments where bats hibernate. WNS causes bats to wake more frequently during winter, burning through fat stores too quickly and often leading to starvation before spring. Pennsylvania has seen dramatic declines in some populations, particularly little brown bats and tri-colored bats, over the past decade.

Despite these challenges, some species, like big brown bats. have shown greater resilience, possibly because they often hibernate in smaller groups or in man-made structures that are less conducive to the fungus. Continued monitoring of hibernating colonies is critical to understanding which species are recovering and which remain at risk.

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The Seasonal Migrants

Not all of Pennsylvania’s bats choose the long winter nap. Some take a very different approach, they migrate. Eastern red bats, hoary bats, and silver-haired bats are the state’s true travelers, heading south in late summer and early fall in search of warmer climates where insects are still plentiful. These long-distance flights can take them hundreds of miles, with some hoary bats documented migrating as far as Central America.

Migration is risky business. Unlike hibernating bats, which spend the winter conserving energy in a safe shelter, migratory bats must navigate storms, predators, and human-made hazards like wind turbines on their journey. But the reward is continuous access to food, allowing them to stay active year-round rather than relying solely on stored fat reserves.

Fall is a particularly exciting time for bat watchers in Pennsylvania because these migrants often pass through the state on their way south. On some nights, bat detectors may pick up an unusual flurry of echolocation calls as these travelers join local bats in the skies. Occasionally, eastern red bats can even be spotted roosting in leaf piles or clinging to tree branches, their reddish fur blending perfectly with autumn foliage.

By understanding when and where these bats travel, researchers can better protect critical stopover habitats and reduce threats along their migratory routes. For those of us on the ground, watching these seasonal movements reminds us that Pennsylvania is part of a much larger web of bat life stretching across the continent.

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Why It Matters

Whether they hibernate through the winter or migrate hundreds of miles, Pennsylvania’s bats play a crucial role in keeping our ecosystems balanced. These small, nocturnal mammals are nature’s pest-control specialists, a single bat can eat thousands of insects in one night. By controlling populations of moths, beetles, and mosquitoes, bats help protect crops, reduce the need for chemical pesticides, and make our summer evenings a little more comfortable. Their impact is so significant that researchers estimate bats save U.S. farmers billions of dollars every year in pest-control services.

Because bats are so important, changes in their populations can have ripple effects throughout the ecosystem. A decline in hibernating species could mean more insect pests in spring, while a drop in migrating species might impact insect control in multiple regions across their travel routes. This is why monitoring bat numbers, whether in caves, barns, or along migration flyways, is so valuable.

Keeping an eye on our Schwarz barn colony is one small but meaningful way to contribute to this bigger picture. When we note how many bats we see at different times of year, we’re helping to create data points that researchers can use to track population trends, study the impacts of diseases like white-nose syndrome, and make conservation recommendations. Landowners can also use this information to decide when to protect roost sites or when to limit pesticide use near known colonies.

In other words, every observation matters. By watching bats closely and sharing what we learn, we become part of the network of people working to ensure these insect-eating allies continue to thrive in Pennsylvania for generations to come.

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How You Can Help

You don’t have to be a scientist to make a difference for Pennsylvania’s bats, there are simple, meaningful ways to get involved right from your own backyard. One of the best things you can do is provide safe roosting habitat by installing a bat house. Early spring is the perfect time to put one up, giving bats a secure place to raise their pups when they return in summer. Bat houses should be mounted on a pole or building in a sunny spot, ideally near a water source, to maximize their use.

Another way to help is by reporting bat sightings to citizen-science projects. The PA Game Commission and organizations like Bat Conservation International rely on reports from the public to track population health and detect changes over time. Whether you see bats emerging from a barn at dusk, flying through a park, or roosting under your porch, those observations contribute to a bigger picture that helps scientists protect these species.

And of course, you can help raise awareness simply by sharing updates from our Schwarz barn bat cam. The more people who watch and appreciate bats, the more support we build for their protection. Encourage friends, family, and neighbors to tune in — especially kids, who often become lifelong wildlife advocates after their first glimpse of these fascinating creatures up close.

Small actions like these, multiplied across many people, can have a big impact. Together, we can ensure that the next generation will still get to see the magical sight of bats streaming out of barns, caves, and trees each summer evening.