An unusual video of mysterious dark objects moving very quickly and erratically over the skies of Denver, Colo., has local residents buzzing.

Fox affiliate KDVR reporter Heidi Hemmat described "an unusual object that appears to launch and land in the metro area." The Federal Aviation Administration (FAA) had no record of anything unusual in the skies at the time, either visually or on radar.

BLOG: UFO Sighted Over Olympics?

The video made news last week when KDVR reported:

"Strange objects caught on camera flying over the city and nobody can explain it. We first learned about these sightings when a metro area man, who does not want to be identified, brought us his home video. He captured the images on his digital camera from a hilltop in Federal Heights looking south toward downtown Denver. … The strangest part is they are flying too fast to see with the naked eye, but when we slowed down the video, several UFOs appear. … Aviation expert Steve Cowell is a former commercial pilot, instructor and FAA accident prevention counselor. He thought he would have a logical explanation, until he watched the video. 'That is not an airplane, that is not a helicopter, those are not birds, I can't identify it,' he said."

Cowell is certainly correct: It is not an airplane nor helicopter, nor birds. He's also right that he can't identify it, though he'd likely have better luck if he was trained as an entomologist instead of an aviation expert.

There are many obvious holes in the spacecraft explanation, not the least of which is that it's amazing that no one in Denver apparently noticed the extraterrestrial spacecraft launching and landing in the skies over the downtown area in the middle of the day.

BLOG: Why Do People Believe in UFOs?

The most likely explanation? A bug or insect, probably a fly or bee.

How Optical Illusions Create UFOs

Assuming that the TV station is not being pranked by the anonymous cameraman, it's not difficult to see how the insects could fool people. It's not necessarily that anyone is stupid or gullible; instead it's an optical illusion created by the camera representing a three-dimensional object in two dimensions.

Cases like this illustrate the difficulty of determining the size of objects when there is nothing nearby to judge scale. The unidentified flying objects can either be interpreted as large and moving very quickly a great distance away, or small, moving more slowly, and close to the camera. The images can look exactly the same.

If you assume that the objects are at a great distance, then they would have to be large, and flying at speeds (and with steering abilities) far beyond anything man-made.

However if you realize that the UFOs look identical to flying insects, the mystery vanishes. And, of course, the FAA tracks aircraft, not insects, which explains why no one saw any trace of them in the skies over Denver.

BLOG: Is the Baltic Sea 'Sunken UFO' a Scam?

Skeptical UFO researcher Robert Sheaffer noted that this is not the first time those flying objects had been filmed: "The 'UFOs' appear at least several times a week [for months], we are told, usually around noon to 1 PM. Most flying insects become more active during the warmest part of the day."

This is also not the first time that flying insects caught on video have been mistaken for UFOs. Indeed the Denver bugs look identical to UFOs in a widely-seen video taken in 2010 at the El Bosque Air Force Base in Chile. Earlier this year that case was widely claimed by bestselling UFO author Leslie Kean to be among the best pieces of evidence for UFOs ever captured on video. If Kean is correct, the evidence for extraterrestrials in our skies may be weaker than anyone imagined.