The bird was found stuck to a glue trap in Park Road, Wembley, alongside another magpie.
A second bird managed to break free and fly away.
The RSPCA said the magpie suffered “terrible injuries” and was put to sleep by a vet.
The traps were banned for public use in July 2022 under the Glue Traps (Offences) Act, but the RSPCA said it has been called out to “several incidents” this year.
RSPCA animal rescue officer Ellie Barnes-Wilson said: “This poor bird suffered terrible injuries, which included having the flight feathers ripped out and a leg skinned down to the flesh.
“The magpie was bleeding from its eyes and abdomen and although it was alive when I went to the location it suffered a slow, agonising death before it was put to sleep to end its suffering.”
She added: “The householder had no idea how the trap came to be on the driveway.
“It may be the birds were caught on it elsewhere and as they’ve been struggling to get free the board has moved across to this location.
“Or it could be someone has been using glue traps to trap rodents and when they’ve found the two birds caught up have tried to dispose of this trap by throwing it there.”
Glue traps are sticky boards made up of a sheet of plastic, cardboard or wood coated with non-drying adhesive, designed to trap rodents such as mice and rats.
The RSPCA said animals, including birds and cats, are often caught on them and suffer horrendous injuries.
RSPCA scientific officer Rebecca Machin said: “Although the glue trap legislation is a really positive step in the right direction, we will continue to campaign until these devices are completely stamped out with a blanket ban on use, not only for the general public, but for contractors too.”
