Steam vent
A steam vent is a hole or slit cut in the top crust of a pie to let steam escape during baking, preventing the lid from ballooning, cracking, or going soggy underneath.
A wet chicken pie filling releases a lot of steam, and with nowhere to go it condenses on the underside of the lid and lifts or softens it. One central hole or a few short slashes cut before the pie goes in the oven is enough; make them after the egg wash so they stay open.
A pie bird does the same job through a chimney while also propping up the centre of the lid. If the vents seal themselves shut as the pastry puffs, reopen them with the tip of a knife partway through baking.