I concluded that the microphone on that particular audiomoth was faulty. I have since acquired 3 further audiomoths (I have 4 in total) and they are all working fine. I hope the microphone on yours is OK and there's another explanation.

Thanks Andy. I think that's it. Just re-visited the page on what the flashing lights mean. I thought that the more red flashes when switching to USB/OFF meant a battery with lots of charge. This is the case with alkaline batteries - but not with NiMH ! Thanks, a slightly embarrassed Richard

Hi Andy, Thanks for quick response. I download the previous nights recordings on to my laptop and delete them from the 64GB Sandisk sd card by reformatting it before each nights recording. So it can't be a full SD card.

Hi Alex, My AMs are version 1.0.0. I have flashed them with the latest 1.6.0 firmware. However I don't get the option of "minimum trigger duration". Presumably this is because they are the older versions? Also I only have three tabs "Recording Settings", "Schedule" and "Advanced Settings" rather than the four that you have. Also won't setting a minimum frequency of 40kHz miss the calls of a number of UK bats that signal at around 20kHz (eg noctules, Leisler's Serotines and barbastelles). I think I will set the minimum to 15Hz in line with Simon's post.

Wow, thanks to you all for rapid help - much appreciated. I am due to make further recordings tomorrow and will make some adjustments in line with your advice. Happily I have two AMs so can do some side by side comparisons until I get optimum performance.