When I was a child, a small, bird-shaped tchotchke perched on my mother’s kitchen windowsill, silhouetted against the sunlight beyond. I recall being told that it was a pie bird, that you stuck it in the center of a pie before baking and that it whistled when the pie was ready. I had all but forgotten it until I received one, almost identical to my mother’s, for my birthday this year. What was the story with these things after all?
After doing a little research, it seems that the main purpose of the birds (also known as pie funnels or pie chimneys—especially in the UK—and available in other, non-bird shapes, ranging from simple cones to elephants) is to provide a vent, allowing steam to escape from piping hot pie fillings during baking, supposedly precluding them from boiling over and making a mess in the oven. A secondary purpose is to support the center of the pie, in much the way that the center post of a tent works, with the upper crust built up and around the shoulders of the birds. I’ve found no mention (or evidence in my recent experience with my own pie bird) of the whistling aspect described to me as a child, though it does seem plausible that some of these guys might whistle a bit when the filling really gets bubbling and steaming.
According to the box in which my birthday bird came, "…in addition to its very useful function, the bird gives your pie a very 'party' look. This is no new gadget but an old-timer which Grandma will probably remember."
In practice, while I agree with the box that there is a certain archaic charm to be had in using a pie bird, I cannot say that I've experienced any particularly 'useful function' or indispensable benefit to using one. I can achieve sufficient filling ventilation by slashing decorative vents into the upper pie crust, and I’ve never really had any trouble with collapsing pie centers—with or without the bird.
Having gained prevalence in nineteenth century England, it seems likely that pie birds are a remnant of the same Victorian gentility that yielded such highly specialized objects as pickle castors and bacon forks—quaint artifacts, better suited to collection than utility.