curious on ultrasounds

["dobroide" dobroide]

noticed a series of ultrasonic calls in a recording I did the other day
at dusk in a scrub/juniper area near Donana, S Spain.

  sound sample
<http://freesound.iua.upf.edu/samplesViewSingle.php?id=3D37231>

  spectral view
<http://farm2.static.flickr.com/1034/761823513_33cf92597c_o.jpg>

15-21 kHz (and above, obviously), they are strong and show a pattern. At
first I thought of a bat, but maybe they were done by an insect (?).
Similar calls appear often during summer so  I would appreciate any
hint.  Just curious.

Regards

D.