Many radar angels are caused by insects and birds. Their detection on sensitive, high-resolution, Q-, K-, and X-band radars has been verified both observationally and theoretically. Since a radar set surveys a very large volume of the atmosphere and maps it on a relatively small dial, a surprisingly small concentration of insects can cause appreciable clutter on the scope. On sets such as the 0.86-cm TPQ-6 (Cloud Base and Top Indicator), a single insect of detectable size in a volume of 100,000 cubic feet of air is enough to fill the scope with return[VIII-4]. Since the guilty insect would be invisible both to ground observers and to the crew of pursuing jets, a flying-saucer report inspired by the radar echoes would remain forever an “Unknown.” Birds can cause substantial echoes on many radars. Large birds at a distance of ten miles can give signals equivalent to those from a medium-sized aircraft at a distance of fifty miles; in fact, even the fading and fluctuation resemble those of aircraft echoes. On radar, a sea gull may cause a return equivalent to that of a quart of water flying around. The radar cross section of the blip may be several times larger than the geometric cross section of the bird, so that a single adult sea gull at a distance of twenty nautical miles gives a very large radar return. As few as eight birds per square mile can completely fill a PPI (Planned Position Indicator) scope with return[VIII-14]. If conditions were exactly right, the birds might be visible to an observer and the source of the angel would thus be explained. But if no one happened to see the birds, the “mysterious” returns could serve as a basis for still another report of invisible flying saucers. Birds have also been responsible for some of the “ring” angels that have been interpreted as fleets of invisible spaceships. In September 1953 several radar sites in England picked up unidentified objects apparently encircling the city of London. They performed peculiar maneuvers including, according to one saucer publication, the formation of the letters Z and U of the English alphabet. How the correct orientation of this invisible sky writing was determined has never been explained. If the letters are turned top to bottom, back to front, or rotated 90 or 180 degrees, they take on new meanings. Scholars might well argue about whether the first giant symbol should be interpreted as a Roman Z, a Roman N, a Greek Ζ, or a Russian И; and whether the second symbol should be read as a Roman U, a Greek Ω, the mathematical symbol ⊂ standing for “is contained in,” or a Roman C lying on its side.