The nursery rhyme "Sing a Song of Sixpence" which has the lyrics "Four and twenty blackbirds, baked in a pie" is based on pie birds. Pie birds are used with both meat and fruit pies, and in addition to helping steam escape, they also provide support to the crust, keeping it from sagging in the middle.
Nowadays, most people just cut vents in their pastry rather than using pie birds, but hard-core lovers of pie claim pie birds create the best crust. Others may enjoy the whimsical look of pie birds or may prefer to have their crust smooth and un-vented.
Using a pie bird is pretty straightforward; it simply goes on top of your bottom layer of crust in the middle of your pie dish, and your filling is placed around it, and then place your top crust over the pie bird, cutting a small hole in the middle so the pie bird pokes through the crust. Pinch the crust around the pie bird to create a seal. And that's it.
Pie birds can be purchased online - click this link to find some online sources.
(Image: Kathryn Hill)