Bats Migrate

USFWS April 29th Bat Fact

How far do Indiana bats migrate? The longest recorded Indiana bat migration was a female, banded at her maternity colony in Michigan. She migrated 357 miles to a cave in Mammoth Cave National Park in Kentucky.

Did you even know that bats migrate? In my encounters with the public, I have met many people who weren’t even aware that bats left their caves at all except to get food, and were amazed that there were some bats who don’t use caves at all and migrate north and south like many birds do to avoid winter temperatures and conversely take advantage of food and resource rich northern forests in the summer.  Indiana bats sit in the middle of that, and migrate to caves in the winter and trees in the summer (I covered that here).

For many of the specific facts I reference here I looked in my Bat Bible, Bat Ecology edited by Thomas H. Kunz and M. Brock Fenton. They collected together and summarized a lot of scientific papers on bats published prior to 2005.

Bats and Birds

Think about it simply. If you had wings and could go wherever you wanted, would you fly to warmer areas in the winter? Now, take into account how hard it would be to fly that distance (how much food and water you would need, and how physically fit you’d have to be to get anywhere), and you have the limits of where flying animals can go. Most people are aware that a lot of birds go south in the winter. So what are the differences between what birds can do and what bats can do?

Firstly, as the bat fact for today implies, female Indiana bats migrate further than male bats do. Most birds migrate regardless of what sex they are. For all bats in North America for which this information is known, female bats migrate further than males. (In Africa, in the species Little Collared Fruit Bat, the males actually migrate further than the females).

Many people are also aware that birds fly many miles at a time before they stop to rest. Bats don’t do this as much, stopping more frequently to rest and find food than birds do. Because they stop more frequently, bats don’t store up as much fat/energy as birds do before they migrate. Birds have been seen to store up as much as 50% of their body weight before they migrate. Bats store much less than that.

Birds also mate once they get to their summer spots, build nests and lay their eggs there. In a previous post I went over that bats mate in fall swarms and in caves, so what bats manage to do is migrate in the spring while they are pregnant. Is this why bats don’t go as far or store as much food? It could be.

Crazy Migration

The furthest flying distance that bats have been known to migrate is 1,243 miles (2000 km). This feat was recorded in the Nathusius’ pipistrelle . Today’s bat fact puts Indiana bats as far less than that.

Nathusius’s Pipistrelle is little and brown like many bats and occurs across Europe.

Examples of bats in North America that migration include the Mexican Free-tailed bat that’s maximum migration has been shown to be greater than 621 miles (1000km) and the little brown bat (Myotis lucifugus) which has been shown to migrate greater than 248 miles (400km).

But We Don’t Know Much

However, we don’t actually know much about migration about many bat species.  Why is this? Time and price. The most commonly used practice to follow migrating bats is by following a radio transmitter signal. Radio transmitter signals broadcasts are limited to only a few miles at most (with the best conditions). In order to track a bat while it migrates, researchers have to climb into planes or helicopters, stick a radio antenna on the front, and fly around in grids or circles to find the bats.  According to Yahoo answers, it costs about $100 per hour to rent a plane (depending on type and how old it is) plus gas plus the amount of time and money it costs to get a pilot’s license (or more expensively, to rent a pilot). Around here it’s about $5000 total cost wise to get a license, and then you have to log 40 hours in the air. Lots of money, lots of time, and most researchers don’t have the funding to do something like that. Even in the private business sector, I only know of one consulting company, Copperhead Consulting, where they have a pilot on staff.

A timber wolf with a GPS collar.

If you know anything about typical research methods tracking migrating birds and animals, you might be aware that researchers are now using GPS transmitters on larger birds and animals. This is how we know that albatrosses migrate as far as they do, or how we know where all the alpha female wolves are in Yellowstone.

The biggest problem is that GPS transmitters are big, and most bats are little. (And many are little and brown.) I think researchers are waiting until we get GPS technology micro enough that we can glue a GPS transmitter to the back of a 10 gram bat without giving it too much weight to fly around with.

That, or a friend of mine suggested that we train little flying robots to follow bats around and transmit back their coordinates.  Either one.