I thought I’d write a bit about bat houses, and I’ll start with my own. As you can see below, it’s showing some wear and tear from the weather. It’s been up over seven years now, and stood through several hurricanes.
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Ducking in and out of the back door the other night, I was surprised to see something small and dark moving by the outside door frame. Probably a frog, I thought. We get lots of them on the glass sliding door during rainy summers like this one.
Then I shined the flashlight and looked more closely – yikes, a bat!
One reason I haven’t posted as often as I would like here is that I was feeling a bit depressed since it seemed that all or almost all the bats had rather suddenly left the bat house in July. The number of bat transitions — bats passing in or out of the bat house opening — had plummeted.
In June, I logged an average of 64 transitions per night, with at least a few detected every night. But in July, the number dropped to 38, and there were long runs of zero or very few ins and outs. Strange. By early this month, things were very quiet around the backyard.
Making graphs is a lot of work, even if the subject is bats (and their songs). And while automated systems, like the bat logger, are a great boon in the unattended collection of information, they can pump out data at an alarming rate.
Bat calls, “songs” to keep to the theme of this blog, are hardly inarticulate shrieks. In fact, bat echolocation calls are finely honed sonar pulses and have a complex and time-varying structure. The audio and rather low-resolution spectrograms in my earlier postings should hint at that.
There’s a fair amount of interesting scientific research out focusing on bat echolocation, in a much more systematic and detailed way. I was delighted to find not only the article below, but an entire on-line journal whose content is completely free, readily accessible, and freely redistributable.
Looks as if my explanation the other day for the high level of daytime excitement by the bats was both too pat and too optimistic.
It turns out that the individual whom I could see so clearly on the landing area of the bat house was not so much protective as in need of protection — this bat was injured and subsequently died.
Our friends the bats are really active today. Normally, they make a bit of noise when they see me walk by the bat house in the backyard, but today, lots of screeching. (High-pitch squeaks that sound a lot like insects — see the audio/video post below).
It started raining hard here last night, just around sunset. You can see the temperature drop during the rain, and then level off again afterward. Notice the peak in calls when the bats first emerge. Usually it is around 8:00 PM, just when it’s starting to get dark. But, and I’m presuming this is because of the rain, the big peak in calls signifying leaving the bat house is delayed until about 10:00 PM, after the rain lets up.
This coincides with what I heard in listening for the counter that tracks bats emerging from, or entering into the bat house. No counts until about 10, but then almost 200 transitions during the rest of the evening. It seems the bats got a late start because of the rain, but made up for it with a really active night later. Rain probably means more insects for food, too.
In future posts I’ll talk about how I count the bats entering and leaving the bat house, and show some data about how this indicator of activity changes over time.
Here’s a graph of bat echolocation calls throughout the night of 5/16. The yellow line shows the total number of chirps produced as the bats navigate and hunt over my backyard and around the bat house. Since each chirp is a whole set of clicks and very short tones, a single call can produce dozens of loggings. The bat detector converts each call into a series of pulses and each pulse gets counted by a microcontroller. That accounts for the high numbers originating from a maximum of about fifty individual bats who might be in range of the bat detector.
The light blue line is temperature, uncalibrated for now. It was probably in the mid 70’s here last night.
The purple crosses are wind gusts, as counted by an anemometer located next to the detector. Maybe it is coincidence, but it does look as if bat activity picked up as the wind died down last night. Interesting.
The X axis is time, starting around 6:00 PM Saturday night, and ending around 8:00 AM this Sunday morning.
In future posts, I’ll go explain the equipment that logs these data.
Here’s a link to some images I’ve taken of bats.
All are nighttime, Infra-Red shots, grabbed from video, using an Infra-Red flash. I’ll be posting more images over time, since I’m always taking pictures, and I’ll post the more interesting ones here.