Sunday, 31 August 2008
It's 6 a.m., a dark, gray morning in late August, the dim light a reminder how far we've gone on the downhill side of summer. Ethel, a border terrier, and I are behind the house investigating a woodchuck scent. There is a dark smudge in the mist above us, and then another. The bats are returning to their bat house, a thin, slatted box, high up under the eaves. Each bat comes in over the roof, makes a dive for the ground and then swoops upward toward the narrow entrance of the bat house. Some slip inside on the first try, some fall back and try again. After a few minutes, the air is still, the last bat home. Ethel and I turn toward breakfast.
I have seen the bats come out at evening again and again. It is one of the joys of living here, watching them drop one by one into the night. But I've only seen them coming home a few times. The bats of evening are the last flutter in a world that is growing still. The bats of early morning have already been engulfed by birdsong, rooster-crow, the stirring of nearly every creature on this place. Their flight is less erratic just before roosting, no longer distracted by an insect in the air. It's as though each bat brings a scrap of night's darkness home with it, leaving the sky pale and brightening. It's as though night itself were being stored in the bat house till dusk.
When the last bat had vanished, I felt almost absurdly alone, strangely vacant in that thin slice of morning. It reminded me of a feeling from the city a long time ago - that moment, after staying up all night, when you can feel the world gathering pace and energy just as you're beginning to fade. Watching those dawn bats, I imagined them punching out of their night's work as they settled, and I felt as if I'd somehow clocked into their schedule. And it seemed that the best use of a dark, gray morning with mist in the air was to go back to bed, only a few feet, and a couple of walls, away from where the bats are sleeping.
(Normally I have a thing about horrible US English spelling, but writing this good deserves to be left as the author intended!)
My website: http://www.plecotus.co.uk/
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Wednesday, 20 August 2008
There are several reasons the Anabat is so good for passive recording. Alternative methods of recording bat calls use audio formats such as .WAV or .MP3, which quickly gobble up available memory in just a few hours. The Anabat records each bat pass as a series of co-ordinates, plotting the loudest frequency every few milliseconds, creating miniscule data files, allowing many thousands of bat passes to be squeezed onto a single 1Gb CF memory card.
The fact that the memory card is held within the detector is a further advantage, but the biggest advantage lies in the way the detector ignores the spaces between bat passes and simply records each bat pass (or other ultrasonic noise) as a new file. The Analook software which comes with the machine allows you to very quickly scroll from bat call to bat call, identifying each pass and creating a spreasheet showing each one, with date, time and species. The time saving, compared with using other software analysis systems, such as Batsound or Batscan is huge.
So the Anabat is great as a passive detector - why is it good for transect work? The designer, Chris Corben, has built in an interface which allows a GPS receiver to be plugged into the detector. Every two seconds the Anabat asks the GPS where it is and records that data on the CF card, alongside all the bat passes it records as you walk along. This means that for each bat you encounter you have a sonogram of it's call, the date, the time and an accurate grid reference.
Anabat SD1 with Etrex GPS velcroed to it and connected.
Well, I've made it sound great, and it is, but there are plenty of problems too (though apparently Titley Electronics, who make the machine have recognised that there is a huge market for the machine and are working hard at improving it):
2. Even with the GPS and Anabat both set to the correct time, an error of one hour creeps in, though it's easily edited out.
3. Becasue the machine is built in Australia, the GPS data is recorded as Latitude and Longitude, using the WGS84 datum, instead of the British OS datum. This means you have to use a utility programme to convert the data to British National Grid and remember to convert the datum, otherwise you can end up with proper-looking grid references, which are simply wrong.
4. The standard Anabat microphone is not as sensitive as it could be. I set the Anabat's sensitivity control as high as it could take and used it alongside a Bat Box Duet. The Duet was clearly more sensitive, which was disappointing, given the Anabat's £1,400 price tag.
Despite all the above, Anabat plus GPS is still streets ahead of anything else currently available. When used together with the GPSU GPS utility programme and the superlative DMAPW mapping programme, bat data can be identified and transferred to a map with incredible ease and efficiency. Unlike Victor wotsit, the chap who was so impressed with a shaver he bought the company, I didn't go that far, but three weeks after buying an Anabat for passive monitoring I'd bought another for transect surveys, despite the high price.
I was on the receiving end of a hard sell for this system at the Welsh Bat Conference and failed to understand why I would want to pay an extra £400+ for this. Titley admit that the PDA screen will cause you to lose your night vision and advise that you should carry it, pointed away from your eyes! The bracket looks utterly Heath Robinson and vulnerable. When I pointed this out I was told that the bit which would break if you tripped whilst carrying it would be the bracket, the cheapest part...at £100!
In fairness to Titley and to Chris Corben, this is an excellent idea and has immense potential. Why plug your bat survey data into a computer when you can take the computer to the bats, but there's a lot of work to be done yet. Meanwhile, like many others, I am waiting with baited breath for the new Batbox Griffin to be launched. It just might beat Anabat at their own game.
Chris Corben's Anabat website: http://www.hoarybat.com/
Batbox, makers of the new Griffin: http://www.batbox.com/
GPS Utility: http://www.gpsu.co.uk/
Alan Morton's DMAP mapping programme: http://www.dmap.co.uk/
And of course, my website: http://www.plecotus.co.uk/
A juvenile Natterer's Bat
Before I next use the trap I'll trim back the plastic lining. The only other problem that has come to light is that the legs are a bit spindly and wobbly. The trap is light in weight, so the legs don't need to be too strong, so I'll delay making any changes to that for now. But at last, eight months after building it, I know the trap is fully functional and effective.