Shepherd's whistle
They are also produced commercially. Simple whistles may cost less than 1 USD in bulk; expensive ones may be quite ornate. They are commonly made of plastic, aluminum, stainless steel, silver, brass, titanium, Corian, jade, buffalo horn and other materials.
Whistler (radio)
Whistlers were first detected during World War 1. On the wide-band spectrogram, the observed characteristic of a whistler is that the tone rapidly descends over a few seconds. This is the origin of the name "whistlers".