(Or more accurately, those things which make noise).
During the 1970's CB radio become increasingly popular among people who used it, in an ever increasing capacity, as a means for social interaction. As such it became tempting to "show off" to other people, when you had some unique talent or gadget which stood out above the rest. When this show-off tendency combined with technical curiosity, the result was often a string of gadgets, which were guaranteed to get someone's attention. The techs who pioneered these devices would often sell them to their friends, and often made a good deal of money as their popularity grew. The irony, of course, is that the innovation of the creator was destroyed by the commercialization of these devices, to the point were they were no longer considered unique.
One of the most common noise maker gadgets to debut in the 70's was called a "birdie". A birdie was named for the type of sound it produced (roughly analogous to the chirp of a bird). These devices varied in pitch and duration depending on what value parts were used. They were used as a cute little identifier, as a means of jamming, or inducing intentional bleed over.
The two most popular birdie circuits in use in my local area were the 556 timer circuit, and the CA-3045 circuit. I've resurrected my hand drawn and yellowed schematics, re-created them in Visio, and included them here for your curiosity. Usually these circuits were installed inside a mike such as a D-104 and "borrowed" their power from the 9V battery inside. Sudden changes in the speed or pitch of the "birdie" was usually an indication of a weakening battery.
The 556 was fairly common, and a variation on the earlier neon tube "relaxation oscillators". When the SW1 pushbutton is pressed, the circuit will emit a ringing sort of chirp, which closely resembles a modern electronic telephone ring (Even if it predated them by 15 years).
The CA-3045 circuit was a little more elusive, at least in the beginning. The first ones to appear in our area were marketed by someone with the trade name of "Ready Kilowatt". Since he was getting something like $30 for something which consisted of less than $5 worth of parts, it was to his advantage to keep the circuit "secret". In that vein, he went to great lengths to rub off all identifying markings from all the parts when he made them. Eventually we figured out all the parts except for the I.C.. Then some time later, that piece of the puzzle was revealed as well. Once the word got out, many people started making them.
When installed as shown, the circuit will emit a decaying "whistle" sound every time the mike is keyed. The length of the whistle is controlled by the value of C1. When S1 is closed, the birdie will "warble", and will end with a whistle tail when S1 is opened again. The speed and pitch are controlled by C2, C4 and C5 or the associated resistors.