Alfred Dines, 20, was sentenced to three months for breaking the 400-year-old law that protects the birds.
Magistrates in Stroud, Gloucestershire, were told that Dines shot the nesting birds on the Stroud Canal at Ryeford, last September. The mute swans were 'peppered' with bullets - one had been shot 10 times, the other 11.
Dines, already serving two-and-a-half years for attempted armed robbery, was prosecuted under the Criminal Damage Act on Property Belonging to the Crown, which states that any swan which does not bear a mark of ownership is the property of the Crown.
Conservationists welcomed the sentence. Ian Rook, welfare officer at the Swan Sanctuary in Egham, Surrey, said: 'We hope this case will do much to discourage this kind of mindless vandalism. It was callous beyond belief.'