The Speedbird device was designed for Imperial Airways in the mid 1930s and gradually started to appear on a few aircraft before 1939. Designed by Theyre Lee Elliot it was adopted by the newly formed BOAC in 1939 and was painted on many aircraft (even some camouflaged civilianised Mosquitos used for diplomatic runs) during WW2.
After WW2 it appeared on all aircraft, first below the cockpit in very dark blue either under or over the aircraft's individual name and later on the tail.
When the dark blue tail appeared around 1956/7 the emblem stayed on the tail, becoming white.
In the 1960s it changed to gold and occupied the centre of the tail.
kept the Speedbird on its aircraft just behind and above the level of the cockpit windows, again in dark blue, until it was replaced by the Speedmarque in the 1984 colour scheme.
The use of the Speedbird call sign goes back to the time BOAC abandoned using the registration of the aircraft for identity and changed to flight numbers. Officially sanctioned in 1958 with the start of Comet 4 service, the changeover took around 4 - 5 years to be universally used.