As reported by Collector's Weekly:
“The Victorians... liked to send out cards with dead birds on them, robins in particular, which related to ancient customs and legends. There’s a famous quotation from the Venerable Bede about a sparrow flying through the hall of a castle while the nobility is celebrating Christmas: The moment from when it enters until it flies out is very brief, a metaphor for how quickly our lives pass.” Apparently, killing a wren or robin was once a good-luck ritual performed in late December, and during the late 19th century, cards featuring the bodies of these birds were sent to offer good luck in the New Year."
Via Madame Jujujive's always-something-interesting Everlasting Blort.
See also: Piebald robin