I don't know man. I'm always interested in new mic companys and their models, but I get skeptical when they don't list qualifying data on the specs.
What's the deviation in frequency response? What is the reference level for the sensitivity rating? [Usually it's 1V at 1Pa, but that's a dangerous assumption if it's not listed.] Those are the kinds of omissions that make me worry. I'd definitely look for some reviews first.
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One serious issue to consider is that you'll definitely need wind protection for Nature Recording. Looking at the shape of those mics I don't know if you'll be able to fit a ball gag and fur cover on those.
For those who can't afford Sennehisers, usually the Audio Technica AT4021/22 (cards/omnis) are recommended. I have a pair of the AT4021's I use for nature/ambient recording and I'm very satisfied.
There is also an older version of the ATs named 3031/32 that you can sometimes find deals on. The specs are more or less the same.
I have the BT 201 3/s and I'm very satisfied, the best thing is that the sound is very clear and doesn't need EQ. Well I'm using mostly on drums and perucssions so don't know how they would act in field recording. Here's a review about them http://recordinghacks.com/2011/01/13/jz-dmk1-review/