Pie bird
A pie bird, also called a pie funnel, is a hollow ceramic vent placed in the center of a pie that lets steam escape through its beak while supporting the lid.
The funnel shape channels steam up and out instead of letting it soften the pastry from below, and it props the lid so it cannot sag into the filling as the pie cools.
Victorian and Edwardian potteries made them as singing blackbirds, which is why the bird shape stuck. Two knife slashes do the same job with less charm.