They're not like the black specs caused by overheating GPUs some Wii users were experiencing. Those specs are clear, stationary black pixels, sometimes flickering.
The ones I see on Excite Truck are moving across the sky, blending (fading) into the background, like a flock of tiny black globs, pulsating their tentacles, vaguely resembling wing flapping.
OMG I JUST SAW THEM!
But I saw them in a Canada level. I looked up really quick and it looked like birds tbh. They were flying together so that's what I think they're meant to be. I donno maybe not.