In regards to Dodotronic's Ultramics:
I believe that the Ultramic may be useful, for your application - For more info, in regards to the Ultramic, from Dodotronic:
You'll have to disregard the mention of it's ability to do unattended recording - As this feature has not been implemented into the SeaWave software yet. In fact, that functionality may be added to the SeaPro software.
The Ultramics do need to be connected to a computing device of some sort. When hooked up to a laptop, netbook, or Tablet PC, the Ultramic works very well.
The way I see it, when it comes to the Ultramic - Is this: If you will be recording in a place where substantial numbers of bats are pretty much "guaranteed" (Pond, Lake or Stream/River) then; the Ultramic would be good. - But, it does pick up and record other ultrasounds as well. Any sound (or noise) that resonates in (or close to) the ultrasonic spectrum will be picked up.
I was very fond of that unit during my testing and review period. One of the things that I noticed, was the noise-free recordings I got (using the D240X & a Zoom H1 Stereo recorder). The TE recordings were very clean (no background noise). Even the heterodyne recordings were clean.
Folks in attendance, will be able to hear the live bat activity (while you are recording it). The speaker Volume setting has no effect on the recordings. Excellent.
The only issue here, would be the higher cost...
The Ciel CDB 305 Dual Bat Detector looks OK - It's not a terrible choice. However, relying on the printed knob tuning won't be extremely accurate.
Something with a digital readout would be ideal (and accurate).
A unit like the BatBox Duet would be excellent, but it is not cheap -http://batbox.com/duet.asp
Having said that; if cost is a major consideration - I would recommend the following unit:
In any case, with a simple digital recorder - It's very portable. Try to get the shortest stereo cable you can find (with 3.5mm plugs on each end). You'll be able to travel all over (and all night) making FD recordings.
A blog post, regarding cables and other small accessories will be coming up in the not-too-distant future.
The resulting frequency division recordings produced, will be a good starting point - In regards to analyzing the calls and creating sonograms. And, let's keep in mind that the Baton comes with a CD containing the Batscan software! Which I like very much, for creating sonograms, etc. As a side note: BatBox Ltd.'s Batscan software is very similar to the (no-longer-available) Spectrogram 16 software.
The resulting FD recordings will be useful. And you'll be able to analyze them, etc.
Well, I hope that helps. I'm probably forgetting something!