Dependent on exposure time, for example 1/30 sec, it could have moved its wings for, say, 10 ms and then holding those wings still. It would cause 10 ms exposure time of moving wings and 23 of wings in place. That would explain the transparency of the blur. I've seen happening it with a frog, exposure time 10 seconds, that hopped after 4 seconds to another place in the image. It appears now cloned, and a little transparent (the jump went so fast it is invisible). Jan-Carel