Tim Birkhead, author of "A Brand New Bird" and co-author of a paper titled "Bird-keeping and the development of ornithological science," says this sort of setup was common. "My guess," he wrote in an email, "is that this bird might be brought indoors at night (on its contraption) to avoid being killed by cats or owls." There are plenty of paintings of birds tethered by a cord or chain like this. "This must have resulted in a lot of broken limbs," says Birkhead. But there's more to the painting than that.
"The Goldfinch" is a work of trompe l'oeil, an optical illusion, so that when the painting is mounted just above eye level, it appears real. The frame of the painting has nail holes in it and a nail realistically painted just over the artist's signature, and it has been suggested that Fabritius actually hung the painting with a real birdcage, to enhance the illusion.
But why a goldfinch?
The European goldfinch is not closely related to our own American goldfinch, though human introduction in North America has led to the two species co-existing. The European version is a bit duller than the eye-popping yellow American goldfinch, with just a touch of that yellow in a spot on its wings. It has a red face and brownish-grey plumage, and thanks to its love of thistle seeds -- along with thorns, thistles are said in the Bible to be the only thing that will grow after the fall of man -- it has been a favorite of Christian artists and writers for centuries. Ornithologist Herbert Friedmann, in his book "The Symbolic Goldfinch," found the goldfinch in nearly 500 works of Christian art, largely from Italian artists.