Myotis Bat identification problems

Myotis Bat identification problems

Visited a known New Forest Maternity roost of one of the UK Bats I have never seen before.. Bechstein's Bat..
The roost location itself is not publicised, rightly so, but I thought I would try my luck in the approximate area. Didn't take long to find a small group of 4 to 5 Myotis bats circling round the edge of Oak woodland just above head height. Now the problems start as Myotis Bats are pretty much impossible to identify by Bat detector alone, all having a long straight (Frequency modulated) echolocation call. 

The only clue I did get was of a decent sized bat, so ruling out the tiny Whiskered, Brandts and Alcatoes'. The behaviour was not typical of Daubenten's that spend their evenings hovercraft style above water, so I'm left with either Natterer's or Bechstein's.

A typical Natterer's is on the left of the attached pic, my bats on the right.. and they look good for Bechstein's with a slower echolocation, and more restricted bandwidth.. these around 35-70, (but it is difficult to calibrate the spectrogram.. you can tune it to whatever range you wish!) The important difference is that it is much smaller than the Natterer's at the same tuning.

Given that Bechstein's is definitely present in the area, I can probably call these with some confidence, and visit the same area next year for better views. Maybe even a photo would be the way to go for positive identification?

A shame though that the Bat detector doesn't give a definitive answer for these bats. Especially when other bat species are so distinct by their echolocation.

#bats