The reputation is old and culturally entrenched, reinforced by opera and by anecdote. When tested directly, offering magpies piles of shiny metal screws and foil rings alongside matched dull objects beside a food source, the birds showed no preference for the shiny items. If anything they were warier of them, consistent with the general corvid response to novelty known as neophobia.
Magpies do collect material, but for nests, and they cache food. Occasional reports of shiny objects found in nests are consistent with opportunistic collection of anything usable, not a specific attraction to metallic sheen. The strength of the folklore substantially outruns the observational record behind it.
Separately, the Eurasian magpie is one of the few non-mammals reported to respond to a mark visible only in a mirror, a result taken by some as evidence of mirror self-recognition. That finding is influential but has proven difficult to replicate consistently, and its interpretation remains contested.