Friday, October 10, 2014

Bat in the Daylight

It was the type of morning when gray clouds replace the entire sky, and the smoky gold of maple leaves seem to replace the sun. After a day of heavy rain, a damp calm had settled into the woods. Nothing moved except the occasional crumpled leaf finally letting go. As I admired the scene from a second floor window, bigger movements caught my eye.

The erratic fluttering of a small, brown creature was confusing at first. Then it flew straight toward my window, only swerving away at the very last instant. In that moment I saw its big ears, tiny head, furry body, and brown-skinned wings clearly – bat! All summer I’d barely seen a bat—not due to their rarity, but to our differing schedules. Now I’d seen bats in the dark, bats at dawn, and this bat in the day, all in the space of a week.

In some cases, animals being active at the “wrong” time of day can be a sign of trouble. One of the symptoms of white-nose syndrome is that bats venture outside during the winter to replenish their stores of food and water, used up by frequent grooming to battle the fungus. Those bats should be reported to the DNR. Some people also worry that bats outside during the day are a sign that they have rabies. That’s possible, but rare. Bats get knocked down pretty quickly by rabies, have trouble flying, and generally go somewhere dark and quiet to die. Plus, only 6% of sick bats even carry the disease.

This bat was flying quite acrobatically, without any sign of poor health. In a series of swoops, dips, darts and quick turns, it seemed to be outlining the route of a new thriller rollercoaster—one that I would NOT like to ride. Around and around the clearing it swooped, catching insects I couldn’t see.

Hunger must be its motivation, I concluded. With lows in the 40s for several nights in a row, and heavy rains off and on, many night-flying insects have been grounded. Plus, the energy needed just to keep this little mammal’s body temperature high enough would have offset any nocturnal calories it could catch. In a canyon in Italy, a type of bat called the soprano pipistrelle has switched to daytime hunting almost exclusively for these reasons. They’ve found an area under the forest canopy with more daytime insects, and without predators.

This little guy must have decided that the risk was worth it, as he (or she) needs to bulk up quickly for migration and hibernation. Bats may also switch to diurnal hunting in early spring, when they are hungry from the long winter, and it is too cold at night for them and the insects to fly.

While the muted sunlight allowed me to see the bat, I could not even begin to imagine how it was seeing the world. Bats are not blind, and can see about as well as humans, especially in low light. Bats use their eyes to navigate around large objects and across the landscape. Of course, their most accurate way of “seeing” the world is through their amazing powers of echolocation.

Insect-eating bats shout out short bursts of sound, timed to their wing beats to save energy. (Fruit bats and others don’t use echolocation to the same degree, but they don’t live here.) Short silences between the sounds allow the bats to hear the echo of information coming back. Their brains—more advanced than the most powerful supercomputer—use the sounds to create highly accurate pictures of flying insects. Although the sounds have to be as loud as a jet plane in order to echo sufficiently off small, soft insects, they are too high-pitched for us to hear. Bats use tiny muscles to plug their ears while they shout, so they don’t deafen themselves.

Those high frequencies give bats a very precise picture of their prey: the size, whether the insects are hard or soft-bodied, their speed and direction of travel, and much more. But the sounds don’t travel more than a few feet. That’s why bats may swoop near you at night, but always veer away at the last second. (They might also be nabbing that pesky mosquito buzzing your ear.) Bats can “see” objects as thin as human hairs, so they can certainly see you, too. Lower frequency sounds would allow for a greater range, but at the expense of accuracy. Plus, if bats shouted in a frequency we could hear, the night would be a whole lot noisier!

Bats don’t just shout; they have several types of vocalizations. In the evening, just before emerging to hunt, bats “chatter” inside their communal roosts. Some of the chatter is low enough that we can hear it, but much is still above our audible range. Male bats of some species sing to attract females or ward off other males. Bats “honk” at each other in mid-air to avoid collisions, and pups call to their mothers.

Echolocation calls come in a few forms, too. There is a “search phase call,” which is composed of slower sounds used to find prey. A “tracking phase” ensues during the chase. Finally, as the bat closes in on an insect, it speeds up the sounds into a “feeding buzz” that can relay almost continuous information about the prey’s location and direction.

I first discovered those sounds while putting together the Museum’s “Nature’s Superheroes” exhibit. USFS biologist Brian Heeringa sent me several recordings of bat echolocation calls, transformed so that they were partially inside the human range of hearing. Catchy rhythms worthy of a late night dance party spilled out of the computer. Our exhibit committee listened eagerly to the tracks several times over, eventually choosing the liveliest calls for the exhibit’s “bat song wall.” You can come in and hear them, too!

Within an hour, the diurnal bat had disappeared. I hope he found somewhere safe to sleep. In his place, a flock of yellow-rumped warblers invaded the yard. They darted about with the same swooping flights. Unlike the bat, though, they perched briefly in between snacks. What were they all eating?

Stepping outside, I soon had the answer. Little specks of fuzz were drifting around -- woolly alder aphids on the move. (Read about them on the NC blog.) A mosquito buzzed my neck. (Come back, little bat!) Even on a gloomy fall day, the air is filled with life.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! The current exhibit, “Nature’s Superheroes—Adventures with Adaptations,” opens in May 2014 and will remain open until March 2015.

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