A pie bird, pie vent, pie whistle, pie funnel, or pie chimney is a hollow ceramic device, traditionally from Europe and shaped like a bird. Pie birds are steam vents that have been placed in the center of fruit and meat pies during cooking since Victorian Times although some historians, such as Robert J. Leigh, claim a much earlier provenance, e.g. the 16th century.
Pie funnels were used in baking pies and prevent the pie from boiling over in the oven by allowing the steam created when the fruit filling or other contents are cooking 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 hence also known as "crustholders". Older ovens had more problems with uniform heating, and the pie bird prevented boilover in pie cooking.
Traditionally they were most often made in the shape of a bird or an inverted funnel with arches on the bottom for the steam to enter, but they have always 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 as birds because of this song, or whether pie vents in fact existed earlier and the song was a reference to them. 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, and also comments on a putative relationship between the similar terms haggis and haggess (another obsolete name for a magpie).
Source: Wikipedia - Pie Bird