To understand why you might send a friend or family member this morbid missive, we must mentally journey back to the 19th century. And no, it was not madness from the arsenic laced wallpaper or tightly cinched corsets. According to Rebecca Baumann at Indiana University, the cards were particularly prominent in 1880s Britain. With the popularity of mourning rituals and posthumous portraits, death was visually present in daily life.
"May yours be a Joyful Christmas."
The image of a dead bird in the snow is similar to the popular “Babe in the Woods” motif of children who are in their mortal sleep in the forest, and may have likewise been a call to empathy for the less fortunate. John Grossman, author of Christmas Curiosities - the cards were “bound to elicit Victorian sympathy and may reference common stories of poor children freezing to death at Christmas.” It’s worth noting that these cards also have imagery akin to the depictions of the 18th-century English rhyme “Who Killed Cock Robin,” that includes the funeral of the slain bird.
"A Loving Christmas Greeting."
However, it wasn’t necessarily such a tempus fugit symbol. Hunter Oatman-Stanford at Collectors Weekly noted that the birds are often robins and wrens, and that “killing a wren or robin was once a good-luck ritual performed in late December.” Specifically, the Irish St. Stephen’s Day on December 26 is known as “Wren Day,” with a traditional hunt of the bird (albeit now a fake one on a pole, although that wasn’t always the case). So receiving a card with the little prone bird, feet curled in rigor mortis, could be meant to wish nothing more than good cheer on the new year.