Pie funnels were used to keep pie filling from boiling up and leaking through the crust by allowing steam to escape from inside the pie. They also supported the pastry crust in the center of the pie, so that it did not sag in the middle, and are occasionally known as "crust-holders". Older ovens had more problems with uniform heating, and the pie bird prevented boil over in pie cooking.
The traditional inverted funnels with arches on the bottom for steam to enter were followed by ceramic birds, and from the 1940s they have been produced in a multitude of designs. This trend has been particularly noticeable in recent times, due to their increasing popularity as gifts and collectors' items rather than simply utilitarian kitchen tools.
The nursery rhyme "Sing a Song of Sixpence" refers to "Four and twenty blackbirds, baked in a pie; when the pie was opened, the birds began to sing" but it is uncertain whether pie vents were designed to look like birds because of this song. The Oxford English Dictionary comments that the word pie itself (in the culinary sense) may be connected with 'pie' as the name of a variety of birds, in particular the magpie.
Pie birds are somewhat hard to find and highly sought after by collectors today. Despite being three birds short of a complete set, this extremely rare flock shown below -made by the Camark pottery company in the 1940s - still carries an impressive value of $8,000.