Alastair McBeath started in one of the last issues of WGN an interesting discussion about a strange effect that he called a 'dark meteor': Sometimes we see objects that move meteor-like in the sky but are darker than the sky background. As possible explanations both failures of the human vision system and real dark objects like flying birds are considered.
In the last few years I saw several of such unusual objects, too. In private discussions during the last IMC many observers supported the idea of dark phenomena from their own observations, hence, there really must be something causing 'dark meteors'. Almost everybody saw them but nobody talked about it earlier because the thing was just too odd. I always thought that it must have been a bird, I didn't believe in errors of my vision. I've never seen illuminated birds against the dark sky as described by Alastair, on the other hand I had no explanation why bats or birds should move like meteors along straight lines.
Yesterday I browsed through our video tapes looking for some special sound effects. Eventually I noticed by pure chance a 'dark meteor' just following a real one and moving as I remembered it from my observations: a fast 'something' darker than the sky background travelling straight across the screen!
I was, of course, really excited and watched the tape several times. Then I recalled the circumstances of the observations: Both DUBKA and me were watching the Lyrids on April 22/23 in a wood near the south end of Berlin. The sky conditions were normal for those circumstances, we had limiting magnitudes up to 6,0 mag. There were no direct lights in the field of view that might have illuminated birds, nor do I remember any sounds from moving animals despite the noise from some near wild boars.
I digitised all frames and tried to obtain a picture of the event. Unfortunately that was much more complicated than expected: The SNR of single video frames is already bad enough, but dark objects against the sky are completely lost in the noise. I could not average the frames which would have improved the SNR but removed the interesting object, too. It needed some heavy image processing procedures (median filter, extreme contrast enhancement, minimum of all frames) until the trail of the object became weakly visible in the image.
Interestingly enough there did not appear a straight line but a trail with distortions. Furthermore the shape of the object changed, in some frames it appeared longish whereas it was round or turned by 90° in others. This implies that I have recorded a flying animal at a low height over the camera system, most probably a bat. It is much too fast for having an astronomical origin, too, the estimated velocity was about 80°/s.
I want to conclude that at least some 'dark meteors' (not to confuse with 'fuzzy' or 'worm meteors') are real objects and not errors of our human vision system. Bats or maybe birds can be seen dark against the sky and might look as if they move along straight lines. We must consider that they appear only for a friction of a second, thus, we see only a small part of their flight path.